I don't watch too much anime, but I do have one to throw in:
Gundam Seed Destiny.
Dear God...how is it that no one mentioned this ending yet? The series started out much better than its predecessor, Gundam Seed. Then, they decided to shift focus away from the true main cast and put all the focus on the old main cast from Seed. This wouldn't have been a problem if there was actual character development. Instead, the character of Kira just sorta stagnates. It is boring. There is no struggle or anything. Kira stays EXACTLY THE SAME throughout Destiny. It doesn't help that Kira and Lacus are regular Mary Sues that are portrayed as the definition of purity, innocence, and righteousness while everyone else not with the Three Ships/ORB is dirty, evil, genocidal maniacs (Alliance) or control freaks (ZAFT).
Meanwhile, Shin, who was supposed to be the main character, is slowly reduced to a barking dog unable to think for himself that alternates between good guy and bad guy depending on the situation. Participating in an assault on the overwhelmingly black Alliance bad guys? Shin is good. Fighting Jesus Kira and his disciples? Shin is bad again.
Finally, we have the actual ending. The actual ending is a nearly 100% repeat of Seed's ending. Cocking Hell, episode 49 of Destiny is made up almost entirely of modified stock footage from episode 49 of Seed! A large chunk of episode 50 also reuses stock footage from episode 50 of Seed. Oh noes, someone fired a powerful beam at Archangel! Good thing Mwu is there WITH THE EXACT SAME STOCK FOOTAGE to save the day again! The first time it was a powerful sacrifice. Second time it was lame and retarded, especially since Mwu didn't "die" the second time...
Then, when episode 50 finally ended, it didn't "end" so much as just sorta...Stop. There was no actual ending or closure. The original ending was so bad they actually remade episode 50 with more footage for the DVD release. Unfortunately, while there is some closure added, for the most part the alternate version of episode 50 was still pretty terrible and did little to relieve the venom I feel for the series.
Ugh...The ending of that series was just a complete mess. I'm a long time Gundam fan and Destiny ALMOST made me lose all faith in anything decent coming out of the Gundam franchise anymore.