I don't agree with this at all. RDJ does an amazing job as Tony Stark. He's phenomenal and he makes the character better, far more likable than Tony has ever been in the comics. In fact, I think that he's had a positive influence in the way Tony is written in the comics and cartoons and the way he is perceived by fans.
I don't think he detracts from the other characters, because Thor, Cap, and Hulk were all great in their own ways in the Avengers movie. Being the funniest does not automatically make you better. To me, one of the scenes that really gets me in that movie was seeing Cap first fight with Loki. Cap didn't need to be funny to make that scene great, it was just an excellent portrayal of the character and his willingness, with no real superpowers, to go up and fight a god in hand-to-hand combat. That's what makes Cap awesome, not one-liners. I've seen the movie twice, and both times, watching that scene, I get that thrill seeing the battle, that feeling of "this is fucking awesome" that ten year old me used to get seeing two great characters fight in a comic book. That feeling definitely repeats itself throughout the movie, and it does so equally with all the characters, IMO.
I think you're doing the characters a disservice if you expect them all to have witty lines. That's not what they're all about, and the movie would be kind of lame if the Avengers were all joking all the time. I think they struck the perfect balance.
I LOVE BLUD BLOOD! - Rob Liefeld