You know, I think that's it exactly. The characters just aren't as interesting. That, and the writing is formulaic.
Almost every other complaint I have against the last two series -whether it's the stilted dialogue, the drab lighting, the pastel-colored civilian wardrobes or the elevator music- also applies to DS9 and to some degree TNG as well. But we'll forgive it as long as the characters are interesting and the story is taking us someplace new.