2. The Character First Appeared In X Men
This is the one thing that will annoy Marvel fans the most. Quicksilver debuted in X-Men no.4 back in 1964 as an X-MEN character. Of course there is no problem with him crossing over to The Avengers universe, I have no trouble with that. But for the character to make sense he really would have to start as an X Men character with his sister The Scarlet Witch, and then he could join The Avengers, not have one in the Fox universe and one in the Marvel universe. And if no agreement between the two studios to share the rights to the same character could have been reached, then Marvel should do the right thing and not use him. We can't really understand why Joss Whedon has fought so hard for these two particular characters to appear in The Avengers 2 when he has so many other characters he could have used instead like Black Panther, Wasp, Ant Man, Iron Fist, anyone. Does Whedon REALLY have a fully legitimate reason to use Quicksilver over other Marvel characters? We'll see.