Keep in mind, I've only seen the first disc of Firefly - the rest are on the Netflix queue.
You're right, the characters are "cliches" at first, although a better term would be "archetypes". That's something Whedon does deliberately, I think, because it happened in the beginning of Buffy as well. (Willow was the "nerd", Cordelia the "stuck up socialite", Xander the "clown") It lets you get a quick grip on the characters so you can get right into the show. But they don't stay cliches - he develops them into real people as the show goes on. Cordy's greatly changed and becomes a much more developed character by the time she leaves Buffy, and changes even more during Angel. I'm betting the same happens on Firefly.
I mean, really, he's got to introduce what, 9 or 10 characters, and their personal subplots and the main plots of the episodes (which also need to be resolved). You just can't do that in 3 hours, or 6 hours. I could reduce the LOTR characters to one-line "cliches" if I wanted to, and they had what, 10+ hours with the Extended Edition?
J