Apart from the films that seem to crop up again and again as disappointments (like the new Star Wars films, Matrix Reloaded/Revolutions, and so on), there have been quite a few critically acclaimed films which have just left me with an apathetic feeling. Like -
American Splendor - I liked the idea of making a film with comic book devices on screen, but the based on true life story just wasn't that interesting.
The Virgin Suicides - A 'does what it says on the tin' film. Nicely shot, but no real depth is shown behind the characters.
Crouching Tiger, Hidden Dragon - Great scenery, but I had no empathy with the characters. And the bit where they 'run' up the trees looked, well, poor.
Though I think the biggest disappointment I had was with The Crow II. A 'classic' bad sequel. Basically reuse the idea, but somehow remove everything that made the first one enjoyable. And I don't think it was caused just by Brandon Lee's absence.