Private Messaging System/Add Friend
Important to keep in contact with your friends.
Non-Boring Backgrounds
You get bored easy training on the same old grass tile and trees .
Price Increases/deceases
Having items that are uncommon which make them expensive but over they become more common deceasing their trading price.
Player verse player

Items that can grow/incease level
This just gives the player something to work for. For example: Silver Armor+4 (level 4 armor - incease in defence)
Different Classes
Knight, Wizard etc etc...
Eye Candy, Unquie looking characters
If everyone looked the same ... thats just pure bordom.
Different Worlds or Areas that look different from the last
For example: Area 1: Forest... Area 2: Snowy
Thats the basics of best features to make people play longer. These are only suggestions.... no flame!