Right, so the original, now deleted answer, had a good source, but was far too simplistic an answer.

So, I'm going to respond to /u/elos_ questions with the objective of giving a far more informative answer.

What do you mean they 'were forced to rely on'? In what ways? What were the Romanians and Hungarians roles in the battle and why were they important?

The flanks of the 6.Armee were in fact held by Balkan allies, such as the Hungarians and Romanians; and their armies by comparison were undeniably backwards technologically to the Germans. A strong case can even be made to their fighting ability - but that's not what Beevor chose to select as a reason for their impact on the battle.

He's right to avoid commenting on their fighting skills, because in many places, these 'backwards' infantry forces often held back against several waves of breakthrough attacks before eventually succumbing to the assaults. The reason for their ultimate failure? A lack of true mobile support from the Germans (who were stretched far too thin to to have mobile reserves) and a decided lack of modern anti-tank weapons. The two go hand in hand; no German mobile formations to react to armor attacks, no static Anti-tank defenses to hold back the tide on location. Its a potent mixture of Strategic failure or inability on the German's part, technological inferiority and lack of mobility on part of the defenders. "Fighting Prowess" factors in yes, but its too simplistic to say "Oh the Hungarians weren't the fighters the Germans were" - its a tenuous co-relation at best.

What advantages? You say later the Soviets had advantage in tanks, aircraft, and artillery pieces. Or am I misreading you?

The original answer failed to go into detail. IV. Fligerkorps bombed Stalingrad into rubble because originally Stalingrad factored little into Case Blue; it was to be denied as a counterattack route to the Soviets, and its industry bombed into oblivion. German forces were supposed to swing South. Hitler's psyche is what forced German units to completely forego their tactical advantadges and fight in the rubble-ruined city.

German advantage, being, of course, and simply put: Mobility; which leads to flexibility. Beevor states that the German generals and colonels could do very little once the fighting devolved to Company, Platoon and Squad level. At these close quarter, small unit battles, the German's main tactical advantages: superior fire support, were greatly diminished. Mobile artillery, assault guns, and tanks, are far easier to separate from their supporting infantry and vice versa, in such environs. There's a reason why urban areas often are encircled rather than assaulted by Armored forces.

German high command (Hitler especially) underestimated Soviet capabilities and no preparations were made in case of a possible Soviet counter attack

This entirely discounts Soviet strategic superiority. The art of maskirovka was first used to full effect during the Uranus counteroffensive. Was the German command ignorant/arrogant towards Russian reserves? Certainly, but that is a Brother's Grimm interpretation of battle. There is no parables, no moral ending, in war. The Germans underestimated Soviet capability, and this is a combination of bad intelligence and skillful deception by the Soviets.

Nipe in a far more accessible manner than Glantz, describes protocols for Maskirovka well. I quote:

In order to conceal their intentions and achieve this type of operational/strategic surprise, the Russians implemented very detailed, codified maskirovka principles...

1. Concealing the actual movement and preperations for the movement of troops and supplies into the concentration area...

2. Strictly limiting movement of these forces...during daytime

3. Camouflage of the assembly positions and dispositions of the attack force.

4. Elimination of battle reconnaissance

...

6. No firing of artillery that was designated for the support of the attack...registration was accomplished by having roving batteries or already sited guns provide this information.

The list goes on, but I'll leave it at that. The Russians had more men, yes, obviously. More importantly, they disguised how many more they had and were thus able to catch the Germans with 'their Panzers (Ayyyy) down' so to speak. This level of Operational Security shows a strategic brilliance that the Germans could not, or would not comprehend.

They had a lot of equipment they received from the US via Lend-Lease including hundreds of trucks which game them superior mobility.

This is good to note; but even Beevor makes it an after thought. Russians never used IFVs in great quantities, and he's far more ready (logically) to acknowledge the distribution of food supplies as the greatest contribution Lend-Lease had on this particular battle. Lend-Lease hit full force around the Battle of Kursk, when Russians began to receive the (thus far utterly superior) M4 series of tanks, which they equipped a handful of divisions with in their entirety.

Cold winter didn't help either

Oh yes...General Winter. The greatest strategist of all! I won't give this too much space; as I've already detailed the very excellent strategic build-up by the RKKA. I will, however, say this: The 62nd Army was hurting as much as the 6 Army when it came to Winter supplies. The author you quote, Beevor, goes out of his way to state that only newly arriving units - that would be those destined to counterattack - received winter supplies; and there wasn't enough to go around to the already committed divisions. This means, the pinning force, the 62nd, was suffering as many if not more casualties then the Germans in the streets of Stalingrad. What a disastrously disingenuous simplification.

In addition to Beevor's "Stalingrad" I used Glantz's Ft. Leavenworth series of publications, as well as George Nipe's "Decision in the Ukraine"