You guys need to take your John Locke consent of the governed claptrap and set it on fire. Skyrim is not the Confederate States of America, and the Empire isn't the U.S.A. The 'legitimacy' of the Imperial government isn't based on the rights of man, it's based on the divine right of Kings. Talos/Tiber Septim is much more akin to Alexander the Great or Charlemagne than George Washington, which is to say that he won his empire the way every Emperor won theirs: By the strength of arms.
So, once you cut away the utterly spurious premise that the Empire's rule is in any way legitimized by a philisophical foundation apart from 'Do what I say or die', then you can start to realize the merits of Ulfric Stormcloak's arguments in favour of rebelling against Imperial authority, namely that the Empire has failed in its one fundamental duty to protect its realm from external aggressors, namely the Thalmor. Ulfric and his followers shed their blood on behalf of the Empire, and won the Battle of the Red Ring, at great cost, only to have Emperor Titus Mede II waste the fruits of their victory at the bargaining table.
As for arguments that Ulfric is a racist, I don't see any justification of that charge. He wants the Nords to be free of an Imperial government he views as weak and corrupt, and he welcomes any outlander willing to cast their lot in with them into his cause (like my Breton PC). If he's not diverting troops to protect non-Nords from bandit raids, it's because he's conserving his troops to focus on his fight against the Empire. When the handful of Dunmer, Khajiits and other non-Nords pony up troops for his revolt, he might have a reason to risk his forces on their behalf. Until then, like any general, he's picking his battles.
So - as it is - neither right has the divine right of kings. Ulfric hasn't won the Moot. The last king that was of the Septim line is dead. The whole thing there is dead.
Next, Skyrim essentially IS the empire - it was the original province. It's not a foreign country that was invaded at one point, like most of the rest.
Winning one battle, whoop dee doo - the important thing is that when you tally up the casualties for the ALL THE ARMIES OF HUMANITY COMBINED (including the nords) it was higher than 50% - that's /disastrous/ - of course they needed a chance to regroup. It'd be really absurd if every single time an army won one battle and lost the rest of the war of the group that managed to win one battle rebelled - obviously one battle is unimportant in the grand scheme of things. The alternative was to keep fighting when it would have lead to you having no army at all, then you'd REALLY be [censored]. Signing away the temporary worship of a god, when you know most people will still do it in private, is no big deal.
They didn't 'abandon' Hammerfell, Hammerfell seceded on it's own, and the Empire deliberately left them a ton of troops by declaring a ton of highly skilled veterans as 'invalids' and discharging them from the legion so Hammerfall wouldn't be [censored] when they had to pull back to defend Cyrodiil. (those troops then being canonically what formed and lead the insurgenceny that prevented Hammerfell from falling.)
The Empire's rules are legitimized by more than 'do as I say and die' they're legitimized by being one of the most just and fair countries around. Enough so that people have grown to expect equal treatment and a general lack of racism, fairly modern ideas, even in this medieval-ish era.
Beyond this, the Empire is the one, last, great bastion of humanity - the Thalmor want humans fighting amongst themselves, so obviously what you should do is do the exact opposite. Unite. Bickering about a temporary ban on public worship you all do anyway, and about SKYRIM FOR THE NORDS HOO at a time when Humanity is on the brink of EXTINCTION is utter idiocy.
The High King was 'weak' insofar in that he was a very young king, 18-22ish, and the thing that really made this sour? He considered Ulfric his best buddy - his hero, his role model. He would have joined Ulfric's rebellion in an instant had he but asked. The whole of skyrim, seceding at once, instead of pitting neighbor against neighbor. Instead, Ulfric challenges him to a duel, and then, to stomp on his dignity even more, and then kills him with a Shout instead of even giving him the dignity of a fair fight, and all this to prove a political point. (the young, unexperienced warrior versus the old,battled-hardened veteran isn't much of a fair fight in the first place.. then he has to use ancient magic to make it even more unfair, too!). That's pretty much disgusting, especially how he talks as if the King was some spineless puppet of the empire, when he was more the protege of Ulfric.
Yeah.. the stormcloaks are racist. They weren't even there first, so it wasn't their homeland. Argonians who aren't Dragonborn aren't allowed in the city walls. Neither are Khajit. They both live on the docks, in poverty and near-slavery. The Dunmer live in ghettoes, where they're regularly harassed and assaulted.
Saying that they didn't 'contribute' to his army is a [censored] reason - That's like saying that any person whose family doesn't join the military is somehow less of a citizen, or that immigrants who don't join the army deserve to be treated like trash, or, to Godwin, that Jews deserved the racism in Nazi Germany because they didn't support the war effort - the whole thing is rather absurd. A trader is a trader - tehy all make the same amount of money for Whitehelm, so cherrypicking on race is pretty much an entirely racist thing - it's not like the Nord traders are part of the army. Are the non-combatant nord merchants who don't give a [censored] either way and just want to make money somehow more deserving of a guard just because some people of the same race support Ulfric?
Skyrim has been long-settled by people of many races, by this point. Some for decades, some for hundreds of years. As the Nords weren't even there in the first place, saying they have some sort of 'right' to decide who gets to live there is [censored]. Using that as an excuse is like saying you'd support a native american revolution today, with the intention of kicking out everyone without native american blood. The whole idea is completely absurd. It's hundreds of years too late for that, and thousands of years in Skyrim.
As far as the guy who is going that the Imperials are 'better' than the stormcloaks, I can see his point when you actually, physically compare the cities they own. EVERY SINGLE stormcloak city is corrupt, while every single imperial city is successful and happy.. except for Markarth. Guess why Markarth is corrupt? Because the ruling family that's hiring murderers to kill dissenters in the street, and have essentially an institutionalized slavery thing going on. Who happen to be extreme stormcloak supporters.