I'm pretty happy with this season as a whole. When I heard the topic was going to be religion my big fear was that they would give Dexter some kind of religious conversion that would later be reversed (which would be totally out of character) but thankfully they've kept him about where he should be regarding the subject. The villain this year is kind of a numbskull but I think that's kind of the point of the character. I think the intern is a pretty interesting character and don't quite get what he's about yet, which is good. The Deb + Dex thing I think is just there to be provocative, and it's fine in that context.
The voice over is still horrible (half the time it's Dexter being Captain Obvious for the sake of really, really stupid viewers) and Harry only makes it worse, since he takes over what would otherwise be important voice over, forcing the writers to use Dexter's narration to impart far less essential (or obvious) information.
I think they've always been like that. I noticed it during my last rewatch of the first couple of seasons. I'd like to think he's intentionally written that way. Not very smart at anything except killing. Sort of a murder savant.