In my humble option there are a number of elements that provide that missing element that keeps you interested
Good PVP (keeps your interest goign when you reach the end)
Ways to show off e.g. Armour huge weapons etc. Look what I have got syndrome
Reason to play e.g good versus evil ideally PVP a good example for me is the Alterac Valley battle in WOW (although poorly executed but right idea)
No Grinding (this should be outlawed)
Varied classes and appearance (in Conan this is just not the case everyone looks the same)
Please add to this list