I think people just have unreasonably high expectations for it. It's the third movie in a trilogy, so I'm not expecting much. The only third movie I can think of at the moment that was better than one of the two movies that preceded it would have to be "The Last Crusade" (I consider it better than "Temple of Doom").

Roger Ebert is really the only critic I give any consideration to. I don't let his reviews determine what movies I go to see, but he's usually right about a film, or at least is more insightful about them. Anyway, he gave it 3 stars out of four, which seems about right.