1. Enemies can stand still for too long (imo)
2. You can push an enemy whilst invisible or using chameleon and your enemy doesn't notice anything, when he should wave his sword about or cast a detect life spell.
3. If you fire an arrow and go behind cover the target knows exactly where you are, when he should only know what direction the arrow came from.
4. You should be able to disguise yourself with a factions clothing.
5. Enemies should notice your shadow.
6. Enemies should sneak.
7. Weight of equipment should effect sneaking ability.
8. Enemies should be alerted more realisticly (by missed attacks, dead bodies)
9. You should be able to knock out enemies(maybe with sneak attacks with blunt weapons and punches).
10. You should be able to drag bodies.
11. Killling all guards that witnessed a crime should lower your bounty.
12. Being seen in sneak mode should make friendly NPCs suspicious.
13. Enemies shouldn't know where you are when you've moved out of sight and they don't know where you are they should behave more realisticly.
14. Magic using enemies should use spells to find you when you are in darkeness or invisible.