1
st.
hardcoe, it makes an RPG game even more RPG and not Action/Shooter.
2
nd.
More and Shorter Quests, gives you a bigger variety and keeps you on your toes and not bored of the same old mission.
3
rd.
Humanoid and Non-Humanoid Companion, having two companions always makes things easier and better.
4
th.
Factions, always makes things more fun.
:D