-Both monsters can specifically climb ladders and avoid "obvious" traps (having a TNT block in plain sight wouldn't work for example, but one that got triggered by a pressure plate would still be effective.), if they allow a closer distance to players (zombies) or a better shot at players (skeletons)
-Skeletons don't fire as soon as possible when shooting a bow. Instead they wait a few seconds to make sure they don't hit something else besides their target.
These AI tweaks would only happen in the Normal and Hard difficulties. Easy would remain the same.
That's all I can think of for now. I'll add more if I feel it's necessary.