Ooh I can think of lots and lots, I wonder if that's because I go in with too high expectations or the fact that I'm too easily unimpressed.
Nemesis, Alton Towers - Probably the biggest disappointment considering all the hype around it and how highly rated it is. Okay it's not total crap, but I just don't find it very enjoyable and it's so meh. It's an excellent concept but I prefer the more standard design.
Thi-three-teen, Alton Towers - Okay, I wasn't expecting it to be great but I really wanted it to be good. Unfortunately, it's just ****.
Ghostrider and the whole of Knott's - Having seen it on a TV show I was really looking forward to it and expected it to be good. It turned out to be incredibly bumpy and uneventful. Knott's as well, I was looking forward to experiencing my first non-Disney/Universal/Busch theme park however it's so tacky, grotty and the atmosphere is putrid. Xcelerator's the park's only saving grace.
Revenge of the Mummy, Universal Studios Hollywood - I rode this a year after I rode Mummy in Florida (which I LOVED) and was expecting it to be up to a similar standard. It was not. Slower, less impressive and just "okay". I'm sure if I'd ridden it first, I wouldn't have thought less of it but I didn't, so yeah that probably clouded my judgement.
Dueling Dragons, Islands of Adventure - It looks cool and the dueling aspects are nice but as rides themselves I think they're a bit pants individually...*
*I hope the use of this ellipsis is not a breach of copyright of Benin's trademark.
Kumali, Flamingoland - Intense? Baby bottom smooth? Interesting layout? No to all three of those. It was rubbish.