I think the only things necessary for a good MMO are the following; anything else is just about individualising your game from everyone elses.
- Trading, PvP, chat, and other multiplayer aspects implemented quite soon
- A large amount of items (hundreds) all with different looks and stats
- A forum, chat, or some other place where trades can be discussed
- Leaderboards or some other way of showing who's "better"
People play MMO games because they want to either socialise, or prove they're better than everyone else. The social aspect will come as you get more players. Until then you gotta have all your multiplayer bits sorted quite quickly.
My MMO is currently missing trading and it's not as active as it could be because of that. But after I introduced leaderboards, activity shot up like a rocket.
In short, sort out the basics first before worrying about aspects such as what characters to have and such.