I'd love an MMO that heavily rewarded grouping up or even required it to progress. Being able to solo 90% of the content is the trend and it's great if you're pressed for time or don't wanna get too involved, but I feel like it reduces the social aspect of the game. I can't remember the last time I was in a group that wasn't formed just to fight one boss or something like that.
I like exploring the game with other people. My friends all became DUNGEON FINDER ALL DAY ERR DAY AND THEN WE CAN RAID ALL DAY ERR DAY types when it comes to MMOs.
GW2's world event system is appealing to me because it brings a buncha people together, but in the end it does still go back to running around alone most of the time.
I'd play the shit out of Wurm Online if it weren't so clunky and technical. It's great just exploring the totally player-built world, and to get anywhere you have to barter with and help out other players. It's a really cool concept, just a little too far on the "need to have the wiki open at all times" side.
I might reinstall GW1 soon

On another note, I've been craving actiony combat like in Neverwinter, Vindictus or TERA lately...but I'm not going back to those games.