I have played Minecraft ever since Beta, I like everything Mojang introduced but for some reason the new mob AI system got a lot of problem which can became extremely annoying in many situations.

1) Enderman walk too fast
-In 1.8, Enderman walk extremely fast. This makes them walking like derpy playeranimation  during Alpha, they flank their hands widely.

2) Mobs often lose line of sight
-New Mob AI has new system which give mobs their own field of view. If a player is hiding behind a wall when getting chased by mobs, the mobs will stop chasing him because they lose their line of sight.
-Can be noticed when fighting Guardians in a room with a lot of pillars. Guardian stop chasing player if player run behind pillars.

3) Speed Hacking
-Happen very often in village. The villager suddenly move like jet when they rush back to house.
-Also, when a mob is running away from hissing creeper, it also move at abnormaly high speed.

4) Laggy Villager
-For some reason, when the villager is in love mode, they move more slowly and looks laggy.

5) No more sprinting kids
-Formerly the villager kids can sprint around and playing tags with other villagers. Now, the villager kid move slowly like adults villager do.

6) Bugged Enderman
See more at here:
http://www.minecraftforum.net/forums/minecraft-discussion/recent-updates-and-snapshots/future-updates/385052-is-everyone-ignoring-the-fact-that-endermen-are