There was little to no actual religious oppression of Talos worshippers until Ulfric Stormcloak essentially started jumping and down going "Hey look a meeeeeeeeeeeeee, I worship Taloooooooooooooos!" Before that the anti-Talos provisions went largely unenforced as the empire was more than happy to turn a blind eye to private or discreet worship.
Err, Ulfric didn't do that. He responded to the Jarl's plea for aid against the Forsworn that had taken over the city. The Jarl/Empire agreed to let them worship Talos if they wished. The Thalmor caught wind of it, and they demanded the Empire arrest Ulfric and his men. The Empire, simply for complying with the Thalmor's demands and arresting the man that saved their land is complete BS. They are the true traitors, both to their founder (Talos) and their citizens.
(Sorry if some of this is a bit inaccurate, not that smart lore-wise)
The Empire refused the Thalmor's demands at first. Then the Thalmor killed every Blade Agent in the Aldmeri Dominion and started killing hundreds of people. Hammerfell only ended up doing fine by itself because suddenly it got to play defensive, which is much easier than offensive. The Empire didn't really arrest or punish anyone either for the Talos worship until Ulfric started parading it around. The Empire was faced with a choice. Let hundreds more innocent lives die, or ban the worship of Talos and for the most part cast a blind eye on it. What would you do?
The Empire was completely on the defensive. They were just caught off guard by the sudden Thalmor invasion. Hammerfell expected it when it happened to them, and thus fought to a stand-still. The Empire could have fought to a stand-still or even won (though at trememndous cost to both sides) had the so-called "Emperor" kept his pair attached to his body. Not only that, but if the Empire had kept fighting, they wouldn't have ost their two most valuable provinces; Skyrim and Hammerfell, for Skyrim's vanguardesque soldiers and Hammerfell's natural-born foot-soldiers and their navy (Which we can assume is probably the force of the former Empire's naval strength). Also with these two very important provinces lost, they are totally cut off from High Rock, thus their source of mages/anti-mages and berserking, bloodlusting Orsimer. Therefore the Thalmor have effectively cut Cyrodiil from her resources that she needs to simply survive, let alone thrive.
The stormcloaks may fight to bring freedom of worship to Talos back, but at the same time, they are cynical, and enslave and discriminate against other races. They have no genuinity in helping the land become peaceful, because also what they're doing is causing thousands more to be killed in an already tedious relationship shared of the land.
Gee, here I thought that being cynical towards the other races of Tamriel was somewhat justified since almost all have impacted Skyrim negatively.
Altmer, Bosmer, Khajiit - The combined Aldmeri Dominion, who have effectively taken control of the "Empire", thus making Cyrodiil their puppet-nation. They also ban the worship of the most iconic being in all of Tamriel, Talos, who is the pride of every true Nord in Skyrim. They hunt down anyone they please and all in all are not very good for the people, regardless of race, in Skyrim.
Imperials - Puppets of the Thalmor as I stated above and enforce the will of them through military force. They stand for all Skyrim i against. They are not valourous, honourable, or loyal. Examples, in order, are as follows: 1) They surrender to the Thalmor 2) They engorce the will of the Thalmor 3) They turn on their own citizens in the name of a treaty that should not have been signed, and was only signed so a fat "emperor" could keep his throne.
Dunmer - Both live in the remains of Morrowind, which borders Skyrim. Dunmer cross the border as refugees that are a plague on their war effort; they refuse to fight for the only group willing to confront an entirely better outfitted and trained military force, yet they want to live in towns and pretend they owe no loyalty. I am a fan of the Dunmer, but I have to agree (although bitterly) with the Stormcloak supporters; the Dunmer refugees are a sap on the system like a welfare case.
Argonians - Not a direct threat, but if they so choose to (which is likely) they could make a conquest to conquer Skyrim as they did Morrowind, seeing the weakened state as an opportunity for a new slab of territory.
Bretons - The Forsworn are Bretons and are waging their own little war on Skyrim. This doesn't do them jsutice in the eyes of the Nords. However if it was possible, I would have Markarth set under a unique form of government consisting of two leaders; one being a Forsworn (or if it is the case, they wouldn't be Forsworn any longer) and the other being a Nordic. They have a reason to hate the Nords as well (as you learn in Cidhna Mine), but still, their impact is negative in the minds of the Nords of Skyrim and more specifically, the Reach.
Only Redguards and Orcs (and civilized Bretons) haven't negatively impacted the Nords. I don't blame them for being cynical racists, but I also don't condone it.
Never have the Stormcloaks enslaved. They discriminate, yes (but I explained why, though this doesn't necessarily make it right), but they are not slave-masters.
The Stormcloaks want Skyrim to be under Nordic rule now that the Empire is not the best choice for them. I don't know about you, but would you give up what you want just so everything could be peaceful? No, because then you would suffer and human pychology dictates that your happiness is more important than others' (to a point)
The tedious relationship of the land is because the Empire is occupying their land. Anyone who is willing to be forced into unity is not going to take it seriously and will not abide by it.