Kevin Murphy: Well we're over the Planet of the Apes, approaching The Phantom Planet, right near the Planet of the Vampires which is right across from the Prehistoric Planet.
Wrex: Right. Because humans have a wide range of cultures and attitudes, but krogan all think and act exactly alike.
Warden: I wasn't expecting that.
Sten: Get used to disappointment. People are not simple. They cannot be defined for easy reference in the manner of: 'the elves are a lithe, pointy-eared people who excel at poverty.'
The Kirk: Is it... an alien?
Alien: I. Am. An. Alien!
The Kirk: Oh, right. So whats's The Thing about you, then?
Alien: It's that we'll try to eat you after a bit.
Leela: But why do you even want to?
Leader: Hey, everybody's got their "thing". We love shipping and handling, all right?
And that makes it tough for the writer to have the critters seem like there are significant personality differences among them. I know, I was constantly trying to solve this problem while writing my drow novel.
That, I think, is, from the author's point of view, part of the appeal of what I call Chinese menu fantasy, where the Tolkienesque band of protagonists has one archetypal elf, one archetypal dwarf, etc. It's much easier to make the characterization work. And I'm not denigrating this approach. I wouldn't dare, now that I've done the diverse band of heroes myself in my dragon thingie.
Of course, there's at least one other advantage to this approach, also. Frequently, much of the point of a fantasy is to give the reader the chance to explore an exotic imaginary world, and by giving him extended commerce with characters who represent many of its races and cultures, the writer facilitates this process.
Wizard: It's not a "Wizard Planet". It's a planet with a few wizards on it.
Astronaut 2: That sounds like a Wizard Planet to me.
Wizard: That would be like calling an ocean "Shark Ocean."
Astronaut 1: Now that's a badass ocean!