I have to agree that the Demon Hunter starts slow, probably more so than other classes, but for some reason I keep coming back to her. I really liked some of the skills I had, and then were changed up/removed from the beta. Strafe and Evasive Fire both felt really good. Vault is a favorite.
Wizard I like, but regardless of the skills, I think the armor progression is the weakest of all the characters. The sets are too similar and there isn't a single one that really makes me say "WoW!" like there is with at least one set on each of the other classes.
Barbarian is a good, solid class. Because of that though, he just seems a bit average. He's sort of the "balanced stats in every category" character archetype.
Monk and Witch Doctor don't appeal much to me, but they may be more of an acquired taste that takes some time to appreciate. They might blossom at later levels, but I haven't enjoyed them as much as the other classes in the beta. I still intend to max out every class in the retail release though.
So I'll probably pick Demon Hunter or Wizard to start, roll a Barbarian second, and then pick the one left over for a 3rd character. Monk and Witch Doctor would be last in no particular order.
Still debating on whether I'd roll a female or male Demon Hunter though. Both are pretty neat, but I'm leaning towards selecting the "canon" gender for most classes. So that would be female DH. Barb I like the male better, though the female design there is really strong. Wizard I would probably never roll a male, not because the female is a great design, but because the male look I find really poor. Monk is a tossup for me - both are very cool. And Witch Doctor I'd probably roll a male - I like the look slightly better, but I'm really not a fan of that at all.