Marvel Heroes is supposed to be pretty good. It went through the same story D3 went through: bad at release and was changed tremendously.
But I'd say you'll find no game as polished as D3, not in this genre. You'd have to broaden your search by the term "MMO" since most ARPGs went that way. There are two games from Asia with potential, which should come out this year. Lost Ark and Lineage Eternal.