The key thing to understand is that Marvel is CHEAP thrifty and they would rather give opportunities to obviously talented but underused directors and actors than spend a fortune on someone that everyone knows is good.
Plus they have a pre-existing relationship with Joss on Runaways and Astonishing X-Men.
A lot of what Marvel is doing is convincing people to do their films for less (and signing them to multiple film deals) because being in a "cool" comic-book movie that makes a ton of dough will help their careers.
In my opinion, this season of Dexter has been one of the great seasons in the history of television. The thanksgiving episode has to go down as one of the all time greatest hours in TV history.. I felt the finale came off as a little rushed.