Mob AI has been improved.
It is now vision and hearing based.
Each mob has a vision cone, extending from their eyes out to the direction they are facing. If the player is not inside that cone, the mob cannot see the player, and the mob will wander around. If the player is inside the cone, but there are blocks hiding the player from the mob, the mob cannot see the player (and will wander).
Each mob has two vision ranges, a regard range, and an attack range. The attack range is always shorter than the regard range.
If a player is within the mobs vision cone (and not obscured), the mob will regard the player (following the players movement), and if the player becomes within the attack range, the mob will chase and attack the player.
If a mob can see the player, and then the player hides behind a block, the mob will remember the player is there for a short time and continue to regard/chase/attack. If the player does not eventually return into the mobs vision, the mob will forget the player was there and return to wandering.
If the player is behind a mob and the mob cannot see the player, if the player then attacks the mob, the mob will turn and return attack.
When a player walks around and is not crouching, the player makes footstep sounds. If a mob is close to the player but cannot see the player, it may hear the footstep and turn to face where the sound came from, allowing it to see the player, and then it's behaviour continues based on sight.
Also a mob may hear other sounds (mob pain, mob death, sword strike etc) and turn to see what is there.
This makes the mob AI much more interesting. You are no longer continually swarmed. You can pick your paths to evade the mobs and have a much better chance of surviving the nightly onslaught.
Here is an example of the vision cones. (for the technical minded among you, they are actually frustums).