Miller had a real gift as a comics writer. He grasped the possibilities of the medium. The original Dark Knight Returns and Batman: Year One are both superb pieces of work, returning Batman to his pulp roots without over-simplifying the character and succeeded in penetrating the popular conception of Batman in a way that hadn't been done since the Sixties TV show. Daredevil: Born Again was excellent as well, re-interpreting the character as more down-to-earth and more human than before.
And then you read Dark Knight Strikes Again or All-Star Batman & Robin (which Miller claimed was parody after everyone panned it) and you see a gun who's so lost the character's core that his Batman wantonly kills (this is a character that, at core, is about a little boy not wanting to see anyone die), kidnaps Dick Grayson and abandons him in the cave to eat rats and turns Superman into a drooling moron (while never up to Batman levels, Supes has always been portrayed as fairly smart).
Miller has become the worst sort of comic writer, the kind who lets his own beliefs write the story. Characters commit actions not because they make sense for the character but because Miller thinks they should. For some reason, conservative writers tend to be especially prone to this, perhaps because they perceive the comics business as much more liberal than it actually is. Bill Willingham's Fables is an example that started and gradually came to incorporate more and more of the writer's conservatism to the point where it was writing the story. I can put up with a little bit of soapboxing but when two main characters in Fables halted the action for two pages to lecture one another (and thereby, the reader) about why gun control was bad, I felt well within my rights to drop the book.
Incidentally (and because I'm a complete geek), good Batman books I'd recommend include Hush and The Long Halloween.
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