In particulair the health/mana/buff block.
I know we have bars on our character but lets face it we can't see it when we pull a map and throw fancy spell effects around :p
But personally I like having that block above my skill bar it's an easier location to glance.
Idealy Id love to have full controle over the UI elements, like in FFXIV etc. BUT I will gladly settle for being able to move the HP/MP/buff bars.