The biggest problem I have with the work of his I don't like is his approach to continuity and the kind of irreverent approach to the characters (in some cases). I'm kind of a fan of irreverent humor (though I hate the term, so it's sort of making me cringe to use it), but it's not what I read comics for (or watch comic book films for). I like these characters, and since I grew up on them, I've formed a more sentimental attachment to them than, say, the characters from Pulp Fiction or Clerks or something I first consumed as an adult. If I someday miraculously outgrow this attachment, I won't be hoping to read clever deconstructions of the comic books I used to like;I'll just stop reading funny books.
In other words, I don't want to read stories about comic book super-heroes by cynical writers who think they're too cool for comic book super-heroes.
This probably sounds like I really hate Millar's work, despite my original statement to the contrary. I don't. I like the stuff that doesn't convey this attitude, and there is plenty of that. As a writer, I think you can leave that sort of thing at the door sometimes. As a consultant, maybe not so much. I basically think they need a fanboy (like Whedon) in that kind of role. Unless I'm reading him wrong (which is entirely possible), I don't think Millar is a full-on fanboy (despite working in the industry).
Then again, the characters I like best are mostly on the Marvel Studios roster, so they're not involved in this. Well, except the Surfer and Ol' Benjy. And Doom. And Galactus. And...
...because he basically deconstructs and reconstructs superheroes with modern sensibilities. That's his gimmick. His Ultimates is basically just taking the tropes of the Avengers that have been ingrained in them, pulling out the main points, and reconstructing it with a different genre (i.e. Ultimates is basically the Avengers in a Michael Bay Film). His fantastic four is basically Fantastic Four in Harry Potter/Hogwarts. His Unfunnies is Cartoon Characters in a Paul Anderson Film. Wanted is Secret Society of Supervillains in Fight Club. Kick Ass is Spiderman in Superbad.
I just finished typing that and I realize that its more like Millar knows how to combine superheroes with movies. That's his schtick more than anything else. He understands the tropes of superheroes and how to successfully merge that with popular film. That's what you need from a movie consultant. Millar knows the edge he has to walk in order to balance all of this correctly and I think he'd do a fine job as a Marvel consultant for film.