I picked:
Customization Options
Casual Content
PvP Content
Social Tools
Grouping
Alternative leveling options
Ability to impact the World
High level content
Greater difficulty
Exploration
Going into each briefly.
Customization Options- Everyone likes to have their own look. This will draw more players to the game and keep them here longer while they try to get the perfect look. Other things can be left or right handed, limps, height, weight, clothing, colors, etc.
Casual Content- Customization would be part of this. Just log in and mess with how your character looks for a bit. Many other things would be small games, easy crafts, fishing, farming, hunting for treasure, etc. Just stuff to do when you are bored of the rest of the game.
PvP Content- make combat more fun, and more players will come. Mechanics dealing with laws (legal and illegal commands) would be a plus.
Social Tools- Leave notes ingame, a public library, voice level tools (from whisper to yell or speech), improved buddy list (rather a buddy tree) for listing friends and enemies (or custom categories), emotes. This will draw more players to the game, as well as keep them.
Grouping- group tasks, group spells, dancing (also social tool), battles, etc.
Alternative leveling options- player to player leveling, book learning, straw dummies instead of rats. Make rat catching an actual job.
Ability to impact the World
High level content- Something important for those high level characters to do, that gives them a reason to get to high levels. A constant skirmish area would do well, with RTS rules.
Greater difficulty- this is worded wrong, but close. I would have stated it as more of a challenge. Making the run to the BD ten times longer would make it more difficult, but no more challenging, and much less fun. Making it half as long as it is, but having to avoid tons of nasties on the way would make it much more fun and challenging.
Exploration- Climbing, everyone love climbing. Real forests. And get rid of the freaking invisible walls.