I love Dexter and I thought this season was the best by far. But with Rita's murder, I think the show has jumped the shark. Not in the cheesy Fonzie way, but I think the show has peaked. I hope I'm wrong, though.
Nah, if the writers are good enough they could easily keep it going as long as they don't fall off the deep end and get stupid (Lost, Heros).
The thing is Rita only became a HUGE part of the show this season, and that's cause they were building you up to her death.
Think of how big the deaths of Doaks (sp?) and Lyla were. Those were also huge characters that took 1-2 seasons of buildup.
Good shows often have big deaths.. look at the Sopranos. I like it because it means the show will not be nearly as Episodic as other TV shows.