If Christian Bale were to get an Academy award nomination for his portrayal of Batman in The Dark Knight (which is unlikely), he would have to be nominated for a supporting role. That pretty well sums up what's wrong with the movie.
In superhero movies the bad guy almost always steals the show. That's just the genre. The bad guy generally has the best lines and is played by a superior actor. Notable examples are Gene Hackman, Jack Nicholson, Tommy Lee Jones, Michelle Pfeiffer, Danny DeVito, Willem Dafoe, Alfred Molina and Liam Neeson. Even in the old Batman TV show the villains were played by an array of great actors.
But even given the typical role of a villain in a superhero movie, in The Dark Knight we see too much of the Joker - and too much of it is fluff. That may be because the actor playing him died and the studio was unwilling to cut his scenes even though he wasn't originally slotted for so much air time. I don't know. Heath Ledger does an okay job, but the core of the movie should be Batman and his relationship with everyone else. Batman gets a bit shunted aside in this one. (Heck, "Batman" isn't even in the title.)
Having said that, there were other things wrong with the movie. I didn't hate it, but I did look at my watch a dozen times and by 90 minutes in (an hour from the end!) I was really, really ready for it to be over.
One thing the director doesn't seem to get is that you shouldn't blow things up just to show off your special effects budget. Satisfying movie destruction can be shocking, compelling, scary, traumatic, redemptive, funny - but it has to provoke some emotion. Probably the world's most satisfying car chase/wreck scene is the one at the end of The Blues Brothers. Dozens and dozens of police cars get smashed. The audience whoops and hollers. That's satisfying. In The Dark Knight many many things get blown up, but I found it all rather ho-hum. In fact, by the end I really didn't care what got blown up or who got killed. I just lost interest.
I wanted to enjoy the movie. I expected to enjoy it. I prefer a well-rounded superhero movie like the Tim Burton Batmans or Ironman, but I can also enjoy a superhero movie that's a bit cheesey or a bit thin. I like the superhero genre in part because they tend to have a dark edge. However, this movie took itself too seriously, and that is the ultimate mood-killer. Its portrayal of Batman tarnished the Batman brand, which is discouraging. In fact, it wasn't even really a superhero movie. It couldn't decide whether it was a summer blockbuster or a serious movie and it ended up just being pretentious and uneven.