I loved the original Guild Wars. There was a decent solo storyline, and then you could play with friends. I've had a hard time making friends on other MMOs...
One thing that instantly turns me off is micro-transactions. I'm happy to pay a monthly subscription if there's enough gameplay to make it worth it (I.e., not just grinding quests like WoW), but when the only way to get the best gear is by paying for it, I'm out. I like games where I can win on my merits and skill and group dynamics, not on how fat my wallet is.
I used to play a MUD (kind of a text-based MMORPG) back in the day. You'd get about 50 levels of solo play before you had to start playing with other people. You COULD play with others, or you could play by yourself. It was really fun! There were ~1000 levels of group play. This was a great way to meet some folks who were also in your level. Over time, you made friends! After you reached about level 900 or so, you could advance to the next tier, at which point you HAD to play with groups.
In this last tier, the entire group (as many players as possible) would go out on a single raid to take down a single monster, and it took a lot of work and it took forever. The monster would drop a few pieces of loot (including one "top tier" item), and there would be a lottery system to determine the winners of the gear. It was a really fun way to play, especially because everyone was a good sport about it. If you already had the gear, you would pass on the lottery, usually. People who already had the gear would go along on the raid just to help other people (and of course to have fun)! It was a very team-oriented culture.
I haven't found a game since then that had that spirit. I really wish I could find something like that.
To me, micro-transaction culture and uber-competitive has completely killed gaming for me. I'm just not interested in pay-to-win scenarios or playing with trolls/jerks.
But a solid single-player experience and a solid cooperative experience where you can play with nice, decent people (especially people you already know)... That would be killer for me.