Exploration and trying to kill stuff that normally need a bigger group with a few of my friends is what made MMOs fun for me... too bad most of my friends now only cares about phat lootz..
Lately I have been more focused on finding a MMO with a fun combat system that makes you feel heroic and is challenging at the same time.. no luck finding one yet... pinning all my last hopes on SWTOR.. let's hope it is as good as the developers says it is.....