Oh good, I was worried that you skipped over season 3. I was fascinated in season 3 by Dexter's relationship with Miguel Prado. It really laid the groundwork for him to lose sight of his "code" with Trinity in season 4. The scene were Miguel killed someone while Dexter watched was so creepy! It was then, I think, that he started letting his guard down a little because he finally had someone he could sort of relate to. If it hadn't been for what he went through with Miguel, I don't think Dexter would have ever tried to get to know Trinity instead of just killing him as soon as he was onto him. He got sloppy and Rita had to pay the price. I think we'll see a different Dexter in season 5. I suspect that he will become cold and withdrawn again, since developing feelings and relationships was what caused him to stray from his rules that had kept him safe for so long.
I agree, Harry probably could have had Dexter rehabilitated, but then we wouldn't have this show!
Also love, love, love LaGuerta and Angel as a couple. I've thought Angel was just a doll from the beginning. Can't wait for season 5!!