I thought AC1 was ok, I love AC2 and also loved Brotherhood. I hated Revelations for a number of reasons:
1. They removed the hidden glyphs on historical buildings, which opened up really cool puzzles. I used to love finding and solving those. Also they meant you had to really explore those historical buildings to find them - which was an interesting way of exposing you to the history.
2. They removed the "dungeons" (Lairs of Romulus in Brotherhood), if thats what they are called, from what I gather. I played Revelations for about 5 hours and never encountered them anyway.
3. The setting was boring to me. Constantinople was bland and uninteresting.
4. Bombs & Guns. I was never fond of guns being introduced to the series, and this entry just takes it to the next level
5. That tower defense ****e. I was constantly trying to keep my notoriety level down, not for any real gameplay reasons - just because I didn't want to be subjected to that ****e mini-game
6. The utterly inane Desmond tetris like/platforming levels nonsense. I couldn't actually believe what I was seeing when I first encountered those levels. What were Ubisoft thinking??
It was like for this game they took a bunch of designers from all sorts of genres, told them to come up with some ideas in separate rooms and then mashed all those ideas into a game without even thinking.
The AC series has been a real favourite of mine, and I was bitterly disappointed in Revelations. I played it for about 5 hours before trading it in.