Most people, from what I've read, say he was both inept and timid, afraid to engage the enemy even when he had superior numbers. It doesn't seem like he was treasonous, per se.

Rafuse, for example, gives his opinion that McClellan simply grew distrustful of those around him, and postponed operations for no reason other than he didn't think the army was ready since the forces had to be prepared under two of his appointed leaders who would work in a concerted effort. This plan, which was the plan for operations in Virginia, was originally going to be put into effect (McClellan thought it up, Lincoln and Winfield Scott had no plans prior to Scott's replacement by McClellan), but was postponed because of the appointments and McClellan's feeling that restructuring would make it better. McClellan, however, didn't seem to have much faith in anyone, and stopped sharing his plans with pretty much anyone who wasn't directly involved. He even kept Lincoln in the dark, and didn't tell him that he had postponed the Virginia offensive.

There was also a three-week period where McClellan was effectively out of commission (he would later say he was not, but this is contradicted by the other accounts of it) because of typhoid fever, which came in bouts. He would, some days, be unable to meet with Lincoln because his sleep came in random patterns, and he was likely not up to form in organizing the troops (this was at the end of December 1861 and start of 1862).

Then came along Stanton, who was the new Secretary of War. Stanton wanted to be an active intermediary between the President and McClellan, and was working with many of the people in Congress who McClellan professed both distaste and distrust for. Like them, Stanton wanted to assume a more active role in the army's movements and get it fighting. This attempt to take some of the power of McClellan, which hadn't occurred yet, probably made him even more distrustful of the administration. McClellan, who initially thought they'd get along great (he wrote "Stanton's appointment was a most unexpected piece of good fortune" on January 18th), found himself in conflict over Stanton's attempts to force McClellan and Lincoln into moving the troops. Rafuse puts it this way, saying that Stanton "...came into office determined to make the army 'fight or run away...the champagne and oysters on the Potomac must be stopped'".

At this point, Lincoln began trying to take a more active role himself in the army's affairs, issuing orders (that were later rescinded) that detailed the line of operations.

Another great source, Rhodes, starts off his review of the first six weeks of McClellan's peninsular campaign by saying this:

If one desires to read a chapter of blunders, or wishes to show how costly it is for a peace-loving people devoted to an industrial civilization to learn the lesson of war, or if he would have an example how decisive events fail of accomplishment wholly for the lack of a great general, let him read the story of McClellan's Peninsular Campaign.

One of the main problems with the campaign is that Lincoln and McClellan really didn't get along by the time that McClellan was poised to break the enemy's line. By the estimate of Rhodes, McClellan did have 58,000 troops to the 11,000 Confederates holding that line, but he chose to wait for reinforcements, writing that "Instant attack would be simple folly". Lincoln told him "I think you better break the enemy's line from Yorktown to Warwick River at once". But because of the profound contempt McClellan had begun to hold for administration officials, and his distrust, he would write to his wife "The President very coolly telegraphed me yesterday that he thought I had better break the enemy's lines at once! I was much tempted to reply that he had better come and do it himself."

But the problem here wasn't treason, by most accounts. The problems were mainly associated with overestimating the strength of the enemy's forces. On April 7th, he was certain that General Johnston had arrived in Yorktown with strong reinforcements, saying that the enemy probably had no less than 100,000 troops.

While it's true that the troops had arrived by the time McClellan advanced, by April 11th they had only 31,500 men...and McClellan had over 100,000. This overestimation is a side-effect of his timid nature. While it could be considered treasonous that he didn't attack when Lincoln wrote to him (on April 9th) that "It is indispensable to you that you strike a blow", he did begin an advance. Unfortunately, by April 17th, as McClellan dilly-dallied in fear of the enemy force, Johnston had taken command personally of a force of 53,000 at Yorktown. At this point, the advantage was dimming.

McClellan was even more mistrustful of Lincoln after he was informed that McDowell's troops (around 35,000) would not be joining him on the Peninsula. Lincoln felt that McClellan had left the capital less guarded than he claimed, and so took the initiative to withdraw those troops from McClellan's command. He wrote in an increasingly paranoid and childish-sounding tone, seemingly thinking that everyone was out to get him. He told his wife that "Not only 'the rebels,' but 'the abolitionists and other scoundrels,' are aiming at his ruin". He then wrote, "History will present a sad record of these traitors who are willing to sacrifice the country and its army for personal spite and personal aims." to his wife as well.

It seemed, from this, that McClellan genuinely believed he had lost numerical superiority. Estimating the enemy forces at over 120,000 at some points, he prepared for an attack using the massive siege batteries he wasted mountains of time to move, around May's start. Johnston, noting this, decided that holding Yorktown was impossible. So he simply evacuated it on May 3rd. McClellan was taken by surprise, since he had planned to attack on the 5th or 6th, but he did order pursuit of the enemy forces. He simply didn't understand how an army with over 120,000 troops would abandon its position, because he didn't understand that they didn't have 120,000 troops.

However, McClellan's order to pursue wasn't entirely helpful. He sent Hooker's division, which overtook the enemy and began the battle of Williamsburg, but gave no orders or general idea of what to do once they found the enemy. Though Hancock's brigade performed well, the rest of the division performed poorly under Hooker, resulting in a Union loss. McClellan decided to pursue them towards Richmond, but did so at a slow pace; around 2-3 miles a day, almost. McClellan was moving so slowly because he constantly complained of the rain, saying the mud made it impossible to move. McClellan even asked an old general of Cossacks who had served in the Russian campaigns against Napoleon how the roads were then. The Cossack replied, "My son, the roads are always bad in wars". Lincoln even began to get fed up at this point with the slowness of the movement, which again seems to be more attributed to McClellan's timidness than anything else, saying, "McClellan seemed to think, in defiance of Scripture, that Heaven sent its rain only on the just and not on the unjust."

Then, when McClellan was ready to assault Richmond, he laid out a plan. His plan, however, was halted by Lincoln. While McClellan acknowledged that he should've taken the James River as the route, since it was safer and clear, he blames the failure to do so on administration officials (that distrust just keeps on comin'!). He asked for more men, and so McDowell's troops were sent (roughly 35,000), but they were sent with conditions. Stanton told him that McDowell would join north or south of the Pamunkey River, and told McClellan to extend his right wing north of Richmond to establish communication. This, McClellan would say, was why he didn't go via the James River. However, this is disproved by what he planned before. He said, in many private correspondences, that he expected to fight Johnston's army between the Chickahominy River and Richmond. He had a full week to consider the plan of taking the James River, before he was notified of McDowell, but said nothing of the sort.

In his letters to his wife, he spoke of his defeat at Williamsburg as a "brilliant victory," and asserted that he had given the Confederates "a tremendous thrashing." May 12 he asked, "Are you satisfied, now, with my bloodless victories?" and May 15 he wrote, "I think that the blows the rebels are now receiving and have lately received ought to break them up."

Quite frankly, McClellan was more likely to be a coward than a traitor. He was afraid to attack, constantly overestimating those he fought, and trying to big himself up because of his pride. He didn't get along with an administration that had no patience for his time-wasting with regards to the many delays and inconveniences, and it's more likely that McClellan was a perfectionist put in charge than anything else, at least in my eyes!

Sources:

Typhoid and Tumult: Lincoln's Response to General McClellan's Bout with Typhoid Fever during the Winter of 1861-62 Ethan S. Rafuse Journal of the Abraham Lincoln Association, Vol. 18, No. 2 (Summer, 1997), pp. 1-16

The First Six Weeks of McClellan's Peninsular Campaign James Ford Rhodes The American Historical Review, Vol. 1, No. 3 (Apr., 1896), pp. 464-472

Generally speaking, McClellan loved his army to the point that he didn't want to use it unless he could win a battle without losing many men. Also, I wouldn't say he was treasonous, so much as he was gullible. During the Peninsula Campaign in 1862, Confederate General MacGruder simply marched one of his regiments back and forth through a clearing to create the impression of a large body of men moving about and McClellan bought it. McClellan also tended to believe the wildly exaggerated estimates of Confederate forces placed in front of him during the war. This lead to hesitation on his part, and even when he won battles, he didn't follow them up. All that being said, he was an amazing organizer of men, and the ability of the Army of the Potomac to endure defeats and achieve victories over the course of the war was probably largely due to the training that McClellan gave them in 1862.
This is a good overview of his tenure as a general: http://www.civilwar.org/education/history/biographies/george-mcclellan.html

Since is something of a subjective question, I'd say it was more that McClellan was just too timid. He excelled as an organizational and logistical general, and was quite proud of the Army of the Potomac he had built, just as the Army was proud of themselves because of him. However, he seems to be one of those officers who is best kept to the rear for training. But, in reality, the comparison between McClellan and Grant might just be something as simple as the difference between Montgomery and Patton. Monty was overly cautious and wanted to ensure that he had the firm upper hand in supplies and position.

An example is the Battle of Antietam. The VI Corps was not employed after the Battle of Crampton's Gap, with one officer writing after the war that it was their general feeling that they would have broken Lee's army had they been employed. From McClellan's standpoint though, his army was licking his wounds and he wasn't going to deploy his reserve in case the Confederates had something up their sleeve.

That last point might point to one of the defining characteristics of McClellan - his paranoia. He was always convinced that he was facing twice as many Confederates as he was - leading to his conservative Montgomery style needing increasingly larger numbers of troops before he would feel comfortable making an assault. The same flaw defeated him earlier in the Peninsula Campaign, not only at the battle of Williamsburg where the Confederates humiliated him by stalling him in front of vacant defenses - but also the Battle of Oak Grove. The commander of III Corps wrote in his diary that Richmond was open for the taking, but inexplicably McClellan got cold feet and temporarily ordered a halt until he was able to personally ascertain that the threat of counterattack wasn't as great as he originally feared.

Regardless of what his politics were in the 1864 presidential campaign for a negotiated peace, McClellan seems to have conducted his duties as well as he was able to with the minor exception that he didn't like the Commander in Chief giving him orders. Perhaps that belies a bit of the Douglas MacArthur temperament.