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.
The kid playing Lex was awesome. The final scene with him looking in the mirror was frightening. He does look like Michael Rosenbaum. If they do not get Michael back, I don't think its all for not mainly due to the strength of the last few episodes.