Ok, just watched all seasons in the last 2 weeks, and here's my bit:
First off, what a fantastic show. Yes, it's sometimes a little unbelievable but what isn't nowadays.
Deb is different, her vocabulary is damn funny at times. I agree she whines a bit too much, but i suppose keeping a macho, tom-boy attitude the whole time is bound to have repercussions.
Rita was portrayed very well, and i think she got the abused wife down perfectly. Seeing anyone that went through an abusive relationship, you would agree that things don't seem right, like when Rita was only concerned about a proper funeral for Paul, that's what bullys in relationships cause to their victims. Great writing. And the end of Season 4, i was  close to tears.
As for Dexter's reactions to her death/other heart-tugging events, i think he is written brilliantly. He has been brought up to not feel, so it's completely understandable when he doesn't show emotion, except for the odd time when he's killing, which is getting more frequent! - again, fantastic writing imo. He even said during Pauls funeral that he doesn't understand the emotion, so understandably he can't portray it. He is getting more and more emotional as the seasons go on though.
I loved Season 4 and 6, and the cliffhanger!!! Grrr!!!!!!
Can't wait for more.