If we extend a bit beyond Trek and allow some serious stereotyping from the average Imperial's viewpoint:
Aslan - TNG Klingons, but with more defined gender roles. (Men fight and lead, Women fix and discover.) Warriors obsessed with Honor and such, some on the Worf-ish side but most like the other TNG Klingons. Remember that Aslan consider Humans to be *capable* of Honorable Behavior, but they're not going to be friendly without the characters doing something to ingratiate themselves.
Vargr - Space Pirates! (Yar! Avast ye mateys!) Less dangerous than Aslan if only because they aren't organized enough to have huge fleets and armies, and won't stay in a fight they're not winning. Vargr will also respect a particular impressive show of force, possibly even befriending players afterwards if the players don't kill too many of their friends. Vargr are also just "normal folks" living around the Imperium, and those guys are going to be like anyone else, though it might still be fun to make them talk like pirates.
Zhodani - Space Nazis or Space WWII Japanese as portrayed in movies. This isn't fair to the Zho (or the Japanese for that matter), but that's how Imperials would see them. Authoritarian, menacing, intolerant and culturally homogeneous. Use them as "black hats", and rehabilitate them later if your campaign goes that way.
Darrians - "Space Elves" in the "Lord of the Rings" mode. The Darrians most players will run into will seem like some of those arrogant scientists from TNG, like the guy with the new warp theory who was with "The Traveler". Darrians are way more curious than LoTR elves, and should generally be seen out on science missions or peddling high tech goods.
Sword Worlders - Romulans with a Norse flavor instead of a Roman one. (It's all Huscarls and "By Odin's Beard!" instead of Centurions and Praetors.)
Traditional/Cultural Vilani - Maybe Betazeds without the telepathy? These guys are generally steeped in tradition and ritual, which you can play for some laughs. Most Imperial humans seem to have gravitated away from the ritualistic behavior of Vland and gone more towards Terran style mercantilism. You could introduce situations with extreme traditionalists who serve many course meals with intricate etiquette rules and opulent surroundings for a change though. If you're familiar with Firefly, the character Inara seems to me to be very Vilani in her application of ritual to her profession. (In fact, the cultural difference that character has with the rest of the entire settings seems very reminiscent of the Vilani to me - a respected relic of another age; not practiced by any but a select few, but known about and respected by everyone.)
Droyne - these guys are going to be hard to deal with. I'd say they're like B5 Vorlons. Mostly they don't interact with others, and when they do it's cryptic or possibly badly translated. ("Black is the suede that hears the future." "Huh? So do you want to buy this laser pistol or not?")
