Well.....
It wasn't hard for me to pick sides. The thing that mainly made it REALLY is for me was the Thalmore. The whole idea of elven overlords who could tell me what i could or could not believe in got my gander up. That and the whole homeland thing. If there is ONE thing worth fighting for in this world, it is HOME.
Of course this is real world feelings creeping their way into a fantasy realm, but, here i could act on them. So im currently level 34, and pushing errr, more like squashing...the Imperials out of my "homeland." (Im playing the True nord / braveheart type) Even before I joined the stormcloaks, i made it a point to kill every Thalmore I saw. The trick is,to provoke them into attacking you first. Then you can slaughter them without any bounty.
Next go around, instead of playing as a "Beowulf" type of character, maybe i'll play a "Maximus" type of character.