Some spoilers from the first episode of season four of Dexter:
Keith Carradine is back, and he knows more than he lets on. I am sure Lundy's business in Miami isn't only to do with the Lithgow's character. Dexter better be careful there.
John Lithgow on the other hand really stole the first episode, despite only appearing for a couple of minutes. The fact that a kill of his is shown within minutes of the new season highlights the importance of the new villain. Hell, the Trinity killer may actually rival the Ice Truck Killer as Dexter's most feared ba(re)d-ass antagonist.
Angel is now boning LaGuerta over Gianna. What a waste of season 3 side-story it seems.
It seems that the writers are keen on Quinn being the new Doakes. Desmond Harrington still can't act though. Personally I hope Dexter off'ed him, and whatever happened to the sub-plot between he and Yuki?
And finally Dexter himself is very tired. I mean very very tired. It seems he isn't able to juggle the life of raising a kid and killing people without getting overtly exhausted, which I thought was kinda silly considering his new super human powers in season two. The cliffhanger at the end of the episode was brilliant though.