Ragnarok Online:
I really enjoyed storming another guilds castle and trying to take it over.
Upgrading your class. Ex. Novice to Swordsman to either Knight or Crusader and so on.
FFXI:
Massive amount of content. (Racked up over 400 days and still never even seen sky, limbus or finished any of the story lines, haha)
Every level felt like a reward.
Gave everyone a reason to fear dying.
Capped Areas. This kept a need for gear of many different levels and also greatly helped out the economy.
Tons of content to unlock.
Jobs were very well thought out and each job felt like something different and new.
One character needed to play all jobs.
Though it sucked having to party to do anything it created a really great community environment.
Partying (Exp and NM fights) actually required strategy and fast thinking when bad situations would arise.
There was lots of content throughout the entire game, and not just endgame.
Phantasy Star Online:
Kept true to its roots and felt like a Phantasy Star game.
Really fun boss fights.
Aion:
PvP content was very fun.