I think it's pretty telling that Robb winning (or at least forcing a stalemate) would not have been too far-fetched had just one or two events not transpired as they did: Lysa staying out of the war, Stannis and Renly splitting their strength, Renly dying, Blackwater, Roob marrying Jane, Edmure redirecting Tywin, Tyrells supporting Lannisters, Roose deliberately losing troops at Duskendale(?), Greyjoys attacking the North, Theon taking Winterfell and maybe even Cat freeing Jaime.
Thinking on it though, the Theon situation was probably the big one because it was quite possibly the worst outcome. Not only do the vikings reject the offer of an alliance, they also decide not to attack the wealthy and pre-occupied West/South but the vast and relatively poor North. Then on top of that, Theon decides to disobey orders, take Winterfell and 'kill' Bran and Rickon, which leads to Catelyn freeing Jaime and Robb losing the Freys.