I used to think that about Leo too, but I've had to admit he can just flat out act. His portrayal of Howard Hughes was one of the best acting jobs I've seen in years. He was much better than Jamie Foxx in "Ray" and should have won best actor. Granted, a skinny smaller guy like Leo playing a big, physical guy like TR is a stretch, but in terms of acting chops I have no doubt Leo could pull it off.
Didn't get to see The Aviator. My husband and I are not DiCaprio fans, but we both thought he did an excellent job in Catch Me If You Can.