I have a few gripes with it, but nothing to make the game less than totally worth it. That said, Assassin's Creed 2 is still the best.
Revelations had a much stronger story than Brotherhood, probably on par with 2's. Seriously great story.
Revelations had a much weaker soundtrack
Revelations is unfortunately short
Revelations has no wanted posters, no horses, no stables. All of these things were better to have in the game.
Bombs are mostly pointless. You could toss out all but like 4 types and not be missing ANYTHING useful or interesting.
Hook Blade is FANTASTIC.
Eagle Sense is great, but I feel that seeing the actual AI path as a line on the ground doesn't sit right with me. Seeing the AI "ghost" walking is good enough.
Not enough stuff to unlock.
Half the fun in AC2 and ACB is running around discovering miscellaneous things and seeing what gets unlocked. Revelations gives you too much money and too much early access to stuff...plus half the stuff you buy is stuff you already bought in Brotherhood. Did I really need to buy another Venetian sword in Constantinople? Really? Why not some local varieties? Why lump all the costume colors in the store right from the start? Why not make me unlock them? Why not sell different colors in different places as a reason to make me explore? Why nothing interesting like rebuilding the Aqueduct?
I like the Den Defense minigame well enough, but I'm not so sure I like how it's triggered.
Leveling Assassins is too easy. Getting them to make your Dens immune to being taken over is too hard.
Not enough side quests.
Why no story time alongside the factions you control? Would gypsies not make for interesting mission threads?
There really should've been waaaaaaaaaaaaaay more time spent with Altair. Him going into exile and then showing up all old....there's a huge time gap there that need filling in....
The game could use a few more grandly huge monuments to explore, but if Constantinople only really has the Hagia Sophia, I guess there's not much to be done about that....
That looks like a lot of gripes, but really none of them are an especially big issue, even all together.