I agree, some of the actors are not too good.
However, the performance of Michael C. Hall is excellent, which is basically the biggest part that matters, as everything is seen from his point of view ( as if you were the character really ). What makes it so interesting, is the theme of a psychopath taught to live life by a code, and later on deciding to become a serial killer. Not for personal gain or just out of impulse, but for making the world a better place ( even if he doesn't know that, or ever feel emotions to understand how ), carefully planning things ahead so nobody would ever find out.
Maybe it's because I've done a lot of research about this disorder itself for a long time.
I don't know, It just fascinates me. I can relate somehow.
I think the series has gone downhill after Season 2 , after sgt.Doakes' death.
It was the most interesting part of the series when sgt.Doakes found out that Dexter was the 'bay harbor butcher'.