About 2 games leap to mind at present, although all of them have been mentioned at some point during the thread.
Borderlands 1 and Fallout 3
For me, Borderlands was a mixed bag of experiences. I remember having fun with it at times (although usually when a friend was playing it with me), but the rest of the time was just a long, dull slog of shooting the same enemies with the same weapons in the same environments over and over again. Then to finally get to the end of it all and that is the ending?
Don't get me wrong, I get what the ending of Borderlands was trying to go for and what it was trying to achieve (at least I hope it's what the devs were going for), but from a gameplay perspective, all of that time sunk into getting to the end and enduring that slow, boring boss fight to not only receive nothing special in the way of loot, but to then have the game say "Ahh wasn't that fun! How would you like to try that again, only harder?!" Just felt like a huge slap in the face and even manages to cheapen the already lacklustre ending.
The pre-Broken Steel ending for Fallout 3 has already received quite a bit of discussion here and for the most part I feel the same way, but continuing with the theme of cheapening endings, doesn't the Broken Steel DLC cheapen the supposed sacrifice the game strongarmed you into making? Since it just turns out that you didn't actually throw your life away and you eventually recover from death in a hospital bed rendering the "destined sacrifice" of vanilla Fallout 3's ending completely moot.