Brandon Stark actually, or accidentally, kills Littlefinger instead of letting him off. Brandon will get killed by the Mad King anyways, so Catelyn will still marry Eddard Stark, without all the "you killed my friend" stuff lying around to prevent it from going off without a hitch. So, while Robert's reign will not be quite so prosperous without Littlefinger there to make gold out of nothing, Littlefinger's plots (which are FAR more expansive than the series has let on so far) will not come to pass. (Though it will likely lead to civil war anyways, since Robert finding out about Jamie and Cersei will from Jon Arryn will undoubtedly lead to him killing one or both, which in turn will provoke Tywin to war. Robert will, however, have a legitimate grievance against the Lannisters, and thus Highgarden is unlikely to go against him (especially if Dorne can then take the opportunity to crush them between themselves and the Stormlands)).
would have liked to see Arya become the "Faceless Man" that I believe she's going to become and end up being the one killing Joffrey. The way Joffrey died was not satisfying at all. After all the things he did a friggin pie ended him. Well, I guess it was Lady Highgarden who administered the poison, but still.
Alright in all honesty as many people have said it i would've preferred Jon to listen to Greywind i mean these wolves are freakin smart people and predicted all these issues yet no one listened to them I never trusted the freys since day one and i believe they should be hunted down because of the red wedding. I've been a stark fan since day one (mostly because i watched an episode first and saw that Eddard stark was boromir and i love that actor) but even that aside. Eddard dieing was fine and i though Robb was a complete and utter badasss. He was as fair as his father and took his soldiers in council i believed his mother is partially at fault for half of their errors and the weakening of his position and i could care less about her. BUT EVEN IF i could not get Robb to live i would of preferred this as an alternative
The "Greatjon" umber escapes and rallys to the north, gathering soldiers and convincing Jon to become King of the north AS I WAS HOPING and he SHOULD'VE been lol. They could've killed the lords of dreadfort with greatjon and pushed the ironmen back and saved Riverrun from the siege its under now.
Other than the obvious answer of "Ned's death" I would choose Robb's decision to send Theon to Pyke because it basically triggers the chain of events that sends everything to s**t.
Theon goes to Pyke > The Ironborn attack the North > Theon takes Winterfell > Theon "kills" Bran and Rickon > Robb hears the news > Robb is distraught and sleeps with Jeyne > He marries Jeyne because HONOR > This pisses off Walder Frey > Red Wedding > Tears > Everything goes to s**t.