Everything in 'Pirates of the Caribbean 3: At World's End' after 'Hoist the Colours.' That was a great opening scene to the film; shocking, compelling, poignant, only to be completely blown by almost everything that followed after it. Okay, that's kind of cheating, but it was really such a good scene that I was looking forward to so much more from the rest of the film.
'The Lost World' once the T-Rex shows up in San Diego. Just who did eat the crew of that ship anyway? The crew dying in confined spaces while the T-Rex was still locked up would seem to suggest that raptors got onboard, yet no follow-up to this is ever made.
I'll agree with others about 'Die Another Day.' When it was Bond being captured, tortured, being a suspected traitor, and having to go on the run to clear his name without any gadgets, I thought it was shaping up to be one of the better Bond outings. Then it managed to turn around and embrace all of the worst traits of the franchise in abundance.
'The Matrix: Revolutions' once it was revealed that Neo had powers in the real world... just because he does. After the ending of 'Reloaded' where Neo uses his powers in the real world I thought for sure they were going to say that Zion and everyone in it was simply part of another level of control beyond The Matrix, but nope, he's just magical like that.
I thought 'Mission to Mars' was a pretty decent film (even with the ridiculous Face on Mars premise) right up until the alien showed up and they flew to that galaxy that's apparently in orbit of Jupiter.
'Transformers' lost me when all the giant robots started doing their gymnastics floor routine in the middle of not-LA, but clearly LA.
Sunshine
I am Legend
Signs
No Country for Old Men