I won't go into a lot of detail excoriating the idiocies of the hardline feminists who allegedly chased Joss Whedon off Twitter. However, here's a refutation of these clowns that I wrote for the Classic Comics Forum:
I have a view contrary to [what some critics have called] the Smurfette Principle.
The first is that, to some extent, the two AVENGERS film are victim to the same problem that dogged the earliest issues of the comic book: what do you do when you've got a group with at least four heavy-hitting heroes? We don't know exactly why Stan Lee chose to dispense with the heavy-hitters and try to sell the title with "Cap's Kooky Quartet." But since Lee's bottom line was always to sell more books, I'd speculate that he knew he couldn't really exploit the Marvel Approach to Character Conflict by using the team-up schema that worked so well for DC. Therefore Thor, Hulk and Iron Man were cast out to allow for more latitude in character-conflict, and for a time the group's biggest hitter was Giant-Man/Goliath, who didn't have his own feature any more and so could enjoy all of his dramatic arcs within the sphere of the AVENGERS comic.
Unlike Lee, Whedon didn't have a choice: the AVENGERS movie-franchise was always meant to follow the pattern of DC's Justice League, exploiting the big-name heroes in both their own features and as part of a group. So Whedon had to include Thor, Hulk, Iron Man, and Captain America (who's not precisely a heavy-hitter but does have an impregnable shield). Hawkeye and Black Widow were spotlighted in their first appearances through connections with SHIELD. They would eventually provide, within the AVENGERS film-franchise, a contrast to all the top-level super-powers, in that Hawkeye and the Widow were closer to ordinary mortals. This is pretty much the same function that "low-powered" heroes like Hawkeye and the Wasp served in the AVENGERS comic-- or, if you prefer a LEGION parallel, Bouncing Boy and Dream Girl.
So if you've got a team of super-gods for which you, the writer, have to provide a credible challenge, you can't take time out to figure, "now how can I come up with something for non-powered Black Widow to do, that allows her to shine?" A single story in a comic can give a low-powered hero a featured position while the big stars are sidelined. A high-FX film like ULTRON can't do that; the low-powered heroes, both Hawkeye and Black Widow were never going to get featured positions.
And that's why I become so torqued off at people like Whedon's critics. They're not thinking about whether what they want is do-able within a particular context; they JUST WANT IT.
Now if the group had written in She-Hulk and given her nothing to do-- that would be the Smurfette Principle.