For me it's not the big features that are missing, but those small ones that would make it fun to play with friends. Options that encourage teaming, such as setting a group mission, would be excellent. I'd also like to see some form of side-kicking (so friends with different level characters could play together), and more instancing so team sizes can be taken into account.

But then I like group PvE play more than solo play, and I appreciate I'm not in the majority.