I find myself disappointed by games that had prequels, or at least spiritual prequels that were better:
Black and White II: I _loved_ the original, so I went right after II and once I discovered that the SP campaign is literally your only gameplay venue (no MP or skirmish) my initial and final reaction said 'go fuck yourselves you hopeless douches.'
NWN: I never played II because I couldn't stand the first, but the fact that I'd played BG I&II beforehand made NWN a huge disappointment. In BG we had a beautifully constructed plot and character development. NWN is a string of find-item quests. Pathetic let-down.
Half-Life 2 + Ep.s: I might very well get some flak for that, but I seriously didn't feel the utter ruthlessness H-L I brought upon me. The original opened my eyes to how awesome a story can unfold in the first person POV, but the sequel feels like a cliche movie lots of times. I also seriously didn't like the vehicle sequences at all. Didn't do it for me. Vehicle sequences should be smooth, fast, and exhillirating. You couldn't move for more than 20 seconds at a time before encountering another stupid, repetitive road-block. Look, road-blocks and chase battles are what vehicle sequences are all about, but you shouldn't have to stop every time you're on the brink of enjoying the speed.
On the bright side, the Orange box had two games that I wasn't expecting too much out of initially that jumped out and pinned me down. Portal, first of all, is Half-Life's real spiritual sequel, and I enjoyed every last second of it thoroughly. The fac that it ended quickly didn't bother me because it ended well, and that is something much more important than game length. Portal was a delightful dessert. Team Fortress II was a feast, and I'm very much in love with it, right now. It is balanced beautifully and I found myself playing and mastering all the classes- something which I don't do in most class-based games.