For me, Arkham City's final boss fights were really anticlimactic. I had a harder time taking out the annoying amount of thugs outside the last level than actually beating it.
Also not the ending of a series but, I remember as a kid being annoyed that Super Mario I think Advance? (the one where it wasn't originally a Mario game, you can choose between Mario, Toad, Peach, Luigi and you throw vegetables at mothertruckers.) You woke up and it was a dream or something even though by just playing the game there was no indication of that ever.
Actually the beginning intro of SMB2 hints that. It's still lame but it is nothing compared to original SMB3's when the princess goes "Sorry Mario, but your princess is in another castle...ha ha. Just kidding".
For me, it seems that the creators ran out ideas how to end the game and decided to put another sequel that literally came out of nowhere, even though the ending looked like it would be impossible to have a sequel
Lol ignore hoggys2much he hasn't even played the damn series, he's just riding on the bandwagon and spewing what he hears other people say
Anyways I was pretty disappointed by Assassin's Creed 3's ending. It was just...lame...Which is a shame because it had such an amazing build up, but Connor's ending wasn't even all that climactic, and Desmond's ending was also bleh on top of that...so that's like... one really long bleh lmao
L.A. Noire had an extremely disappointing ending as well.
Well, its not a final series, but the current final game thusfar; Legend of Zelda: Skyward Sword. They tried to shoehorn so much just to make it a clear prequel, when all the other games swirled around in a vague mist of timeline.