Although this thread hasn't seen any activity in nearly 16 years, it's still a great topic that I haven't seen anyone create a new one on yet, so I think it wouldn't hurt to bring it back. Anyways, the one movie that always comes to my mind is:
Lilo & Stitch
I can remember being pretty excited about it back when it was new, but upon seeing it, I was pretty disappointed. I don't remember what my expectations were at the time, all I know is it just wasn't what I expected it to be and I ended up not caring for it much at all afterward. Not even watching the movie again several times afterward could change my mind about it and seeing the TV series and its sequel didn't either. Plus, I didn't really like Stitch as a character, I thought he was just weird, creepy, and kind of annoying. I didn't care for his alien friends either.
I will admit though, I am going off memory here, I have not seen the movie in a long time, but I have seen some clips of it on Youtube. While seeing clips from old shows or movies from my childhood can be a nostalgic experience for me and potentially give me a renewed interest in it again, Lilo & Stitch is not one of them and I just don't have much interest in it. Though I will give it the benefit of a doubt, I may watch it again sometime since I have Disney+ and it's possible in retrospect, it might be a lot better than I remember, but consider my expectations to be fairly low.
I don't think it's a "bad" movie, but when I think of disappointing movies, this has always been the first thing to come to mind for me ever since I saw it. That's all I can think of for now.