For me the most important aspect of a RPG are the skills, spells, abilities, feats or whatever you want to call them.
I know they aren't really a requirement for a RPG, but it really gives me the feeling my character is actually learning things, becomes stronger etc.
I find RPG's without skills, upgrades, ... less immersing(doesn't mean they can't be fun, I just like to let my character learn some new tricks :p).
Another important aspect is the story. I don't care if it's cheesy, I just want the feeling that I'm part of the world and my actions affect the world around me.