Roy's problem isn't that he's a complete dick - if I recall, that was more the case with his British counterpart. It's that he's sort of clueless, uninteresting, and, key in the world of the Office (and in life, to some degree, I'd argue), not operating on the same humor level as those around him. Specifically, he's not like those characters with whom we're meant to relate most fully - Jim and Pam, and, maybe to a lesser degree, Ryan.
As with the British series, you're rooting for Jim/Tim and Pam/Dawn to extract themselves from this boring, oppressive existence, and hope that they can do it together. Nice or not, Roy basically represents the appeal of settling for the mundane.
Karen, on the other hand, is an animal we didn't see in the British series. She's clearly ambitious, at least in the context of the corporate world, and has more to offer Jim than Roy does Pam in terms of empathy and shared interests, but she obviously still doesn't quite "get" Jim.