I actually thought Crystal Skull was ok. It wasn't that memorable, it was convoluted, and there were some questionable elements, but it entertained me while I was watching it. Though, admittedly, I don't have a strong attachment to the series, so a "just ok" entry isn't as disappointing to me as it would be to some.
Edit: For the sake of discussion, here are a few more:
The Desolation of Smaug: This movie had almost nothing to do with its source material. The new additions to the story were so poorly written that I found them to be an insult to Tolkien's legacy. I think the root problem is that they tried to make the stakes bigger, to make it feel as epic as the LotR movies. The problem with that is, The Hobbit has much lower stakes than LotR, and trying to raise them dilutes the story, turning the focus away from Bilbo. I also thought the CGI wasn't very good, the score has no memorable moments, and the fight scenes were stupidly over-the-top. I could go on, but I'll leave it there. I hated this movie.
Man of Steel: It was a fight scene. The first half of the movie seemed promising, but then any character and plot developments were aborted so there could be an hour long fight scene. I have zero problem with fight scenes in and of themselves, in fact, I don't know what I'd do without them, but when they end up being the only real substance a movie has, there's a problem. I also didn't like that they didn't show Superman trying to save anyone directly, or even trying to lead the bad guys away from populated areas, as he did in Superman II.
The Day the Earth Stood Still (2008): The original is great. The remake has wooden acting and tries to forcibly stuff its message down your throat, not to mention a nonsensical plot.
Penny Serenade: I normally love Cary Grant movies, but I found this to be maudlin. It also treats its heavy subject matter too lightly--spoilers!--the adopted daughter of the main characters dies, and they almost divorce out of grief, but then the adoption agency tells them that now there's a little boy just like the characters had originally wanted, and that makes everything better. Yup.