Not spoiler heavy, but there are a few; those who wanna keep the mystery, refrain from reading
The Empire, because of the uncanny resemblance to the Romans. I always had and always will have unwavering respect for the Roman Empire or the Republic, but aside from that, no matter how I analyze it, the Stormcloaks just strike me as xenophobic, short sighted idiots. It's not even a choice of bad and worse.
Ulfric did in fact murder the High King and escaped afterwards like a whimp. According to northern tradition, if he indeed was acting out of honor, he should have resorted to martial might in the face of his opponent, and also respect the way of the voice, by not using it for personal ego or gain. Branded him a cowardly thug right there.
I mean, the Empire just barely staggered away from a very costly war with the Aldmeri Dominion. We nearly got wiped out, and a reprieve was badly needed to regain strength for the impeding Aldmeri treachery, when they decide to mobilize and finish off the bleeding Empire and its provinces, to declare Elven supremacy.
The cost of banning the worship of Talos/Tiber Septim as a God, as opposed to a brilliant Emperor and strategist is a small price to pay, in the face of complete destruction. How could Ulrfic and his Stormcloaks even hope to withstand the Elves alone, when The Empire, in a joint effort with a united Skyrim, Redguard and other provinces failed? It's numbers, strategy, and a strategic choke hold on the Empire, should the elves gain control of Skyrim. A perfect flank of Cyrodil.
ysgramor always battled the elves, and as a Nord companion, I see no reason to cease that fight, given the aspirations of elves in the active service of the Dominion.
And my visit to Windhelm just sealed my decision. If the way Ulfric runs his city is a preview of his rule as High King, then I'll do anything in my power to see him executed. Not to mention the unpleasant tension in Windhelm, due to the racism and hate exhibited by the short sighted Nords there, even though I play a Nord myself.
But that's just my opinion...maybe watching this thread, I see a good perspective to give me a reason for a Stormcloak playthrough