Guild
It is sad that players can only form parties to play together. When the server refreshes the party crashes and you need to make another one. With guilds system you could be able to play with your friends all time.
Storage
Storage system would be very useful ingame. You could deposit your old items and for example give to your friends who just started. There could also be guild storages to deposit gold and items useful for other players.
Enchanting
Enchanting items is what almost every MMOs have. You could for example collect special stones or materials and use them to enchant your weapon at Blacksmith, of course there would be some enchant success rate. Your item would be for example Steel Axe +1, Steel Axe +2, etc. The higher lvl, the more damage/armor it would have. At some lvl of enchant it would glow.
Crafting
It would be nice if best items from the game would be from crafting. You would collect materials and special recipe which would allow you to craft the item. There would be craft success rate, like at enchanting. Best recipes would be collectable from the best bosses in the game.
Friends system
A special window with your friends list, you would see their nick, rank, class, online status and actual position.
Auction House
Like in WoW, there would be special NPC which when talked to would open a window with others players auctions.
I didn't mention PVP and skills, because I think that you will add it soon. It was in beta, but you removed it : (
Sorry for my English, I hope I am understandable enough.