I think it ended a little abruptly - it could have been a couple of episodes longer, better showing Dexter's letting the Dark Passenger take over completely, and showing Harrison's conflict over that. Yes, Dexter's willingness to kill innocent people in order to protect himself was made clear, but it could have been taken just a little further.
I did not care for the actor playing Harrison. I think he was the weakest lead, and I hope they don't base a spinoff/continuation on that character.
I thought using Deb as Dexter's hallucinatory replacement for his father was masterful. Does Jennifer Carpenter age? Seriously, she looks identical to Season 8 Deb. Batista's cameos were wasted, though. Neat, but he needed to show up at the end in some way.