Oh man, this question has the potential to explode in terms of the scope required for an answer, but I'll do my best. Maybe it would be easiest to break this into 3 digestible points: Lincoln's foreign policies, his military appointments, and his domestic political maneuvering.

First, Lincoln's foreign policies. Lincoln knew that the best chance the South had to achieve any kind of victory in the ACW (American Civil War) was via foreign recognition. To this end, Lincoln did everything he could to incentivize Europe to stay out of things. The three biggest moves Lincoln made in this regard that had a direct (positive) influence on the Union's ultimate victory was his handling of the Trent Affair, his insistence that food staples keep heading east, to famine-stricken Europe, and his decision to issue the Emancipation Proclamation. With the Trent Affair, the Union had intercepted two Confederate diplomats on their way to Europe in 1861. Since they were ostensibly spies (or at the very least, admitted traitors), the Lincoln administration sought to hold the men. The Brits threw a fit since the Confederate diplomats were on a British charter when captured, hence holding the men was an affront to English sovereignty. Now, Lincoln would have been in the right to hold the men, yet so as to keep England pacified (they threatened to send troops over as a formal show of protest), Lincoln let the diplomats go, and remarked "one war at a time." This canny understanding of the larger issue at play (ACW) and geopolitical necessities is a good example of Lincoln keeping his eye on the ball right from the start of the conflict. Second, the South hoped to pressure Europe into recognition by way of the cotton shortage they experienced due to the Union blockade. As luck would have it, what Europe really needed in the 1860's was wheat, not cotton, as they had a dreadful crop famine during that period. Lincoln saw to it that the Northern blockade remained iron-tight, and wheat kept flowing east, which only further dissuaded the decision makers in France and England not to upset their food distributor. Lastly, the Emancipation Proclamation had a huge impact on public opinion in England and France, where anyone vocal about supporting the Confederacy then had to "side" with slavery (this is a simplified break-down of a monumental moment, but it gets the point across).

Now, as for military leadership, Lincoln's record is a bit more spotty. The guy appointed some good generals, but he also placed some dreadful ones in big roles. Through it all, though, you can say this: Lincoln was a hands-on micro manager, and although he took advice to heart, he wasn't afraid to go against his cabinet's recommendations to back a general that he saw as useful. The best example of this is Lincoln's insistence that Grant be kept in command throughout the war, and especially in 1864, when he was appointed head of the Army of the Potomac. He also trusted in the opinion of these men, like Grant, who insisted other good generals like Sheridan and Sherman should be given free reign to lead and plan campaigns as they saw fit. Lincoln also took the time to place (and support) key members of the "back stage" generals, like Herman Haupt, who worked miracles with bridges and railways. Lincoln's best attribute in this regard was that he was not afraid to make mistakes, and that he learned from the ones he did make. Lincoln understood "the math" of the conflict, and as time went on, he got better at identifying the right generals who also understood it, and would help win the war.

Lastly, Lincoln was a political machine. Early in the war, he used all his Executive influence to keep border states like Kentucky and Missouri from slipping over into Confederate hands, and understood how to juggle political necessities (like Gen. Butler) with the practical necessities of the job. Fighting the war meant that Lincoln had to play a political as well as military game, and although there were some missteps along the way (most of them military), Lincoln never wavered in his total, 100% commitment to preserving the Union.

[Sources: James McPherson, 'Battle Cry of Freedom'; Bruce Catton, 'Mr. Lincoln's Army'; Jay Winik, 'April, 1865']

To be honest, I think one of Lincoln's key decisions was to stand back and let Grant fight his war without interference. In the first two years of the war, Lincoln kept saying that he just wanted a fighting general, but if you look at his conduct in the first years of the war, he keeps interfering for political and other reasons.

During the Peninsula Campaign, his pressure keeps Union soldiers defending the approaches to Washington when they could have been more advantageously deployed. He encouraged a focus on eastern Tennessee, which he believed (partially correctly) was a Unionist hotbed and would support the Union if soldiers could reach it. He arranged for attacks on Texas ports and encouraged the disastrous Red River campaign.

His actions on the home front were critically important however, as he managed to keep the North united behind the prosecution of the war despite loud voices calling for compromise and peace.