Ah, yeah, I hate it when animes are rushed/too slow/doesn't live up to the manga lol
I don't think Mai-HiME was rushed though, I just think the ending sucked lol (totally ruined the whole story it was building up on -__-;; )
And as for Gundam SEED, I used to be such a big fan, but now that I look back, I don't really think it's as awesome as I used to think it was.. definitely not on the best list for me

I can't really think of any anime specifically off the top of my head..
I do know of some anime endings that left me quite unsatisfied..
Shaman King's ending was a total "

" I was quite unsatisfied.
Princess Tutu is a really good anime, and even though I did not like the ending at first, but it was very justified. The ending didn't ruin my overall impression (It's a great series)
D.C Da Capo just sucked for me. I had never completely stopped watching a series until this one came up.
I think a series that ended good for me is Escaflowne.. I really like the way it worked out, though the ending is a little bit bittersweet, it was nice. One of the few endings that left me satisfied..
These days, I just go for manga over anime.. usually manga has the side- and back-stories, therefore makes you more attached to the characters, so I don't really watch a lot of anime now..