I prefer to choose my faction first and worry about race later. I don't think I could enjoy the game too much knowing I am a pawn in a machine called tyranny and oppression. I think Gyoin said it well with "It's a bit "Master Race" nazi styled." although the Dominion is a bit more inclusive. The Exiles being your typical freedom fighters are not that special either, but I would probably feel better knowing I am fighting against tyranny rather then to achieve it. I feel that in WoW the factions were not as black and white and you could argue that either side were the bad guys trying to oppress each other. This is more like Star Wars where you have the obvious evil and the hero's trying to stop it. Making a real life comparison I am going for Egyptian protesters going against Pres. Morsi and the Muslim Brotherhood. Yeah that is correct, I just compared the Dominion to the Muslim Brotherhood.
I also think the Exiles have better back stories for their races, the Dominion race back-stories are not as interesting or moving.
Human/Cassian have the same back story but the humans come out as more noble thanks to Serrick Brightland.
Granok I feel are the most interesting, they fought, were defeated, rebels fight again, win, get the Dominion escape off their planet and what do they get? Kicked off their own planet for not following some lame tradition, back-stabbed by their own people.
This story really grips me because I am often in conflict with traditions and cultural differences.
Aurin are cute but I think their back-story is lacking a bit.
Draken seem rather plain too, they love fighting and are loyal to the Dominion because they offer them epic poaching opportunities.
Mechari, sigh... Worst back-story in the game.... Build by the Eldan and are now programmed to blindly obey the Dominion.
Ok, now like Curtech said, lets debate.