Most new MMOs these days allow a significant proportion to be played solo.
You may think this goes somewhat against the very idea of MMOs, but there are players who don't want to have to rely on others and simply prefer to make use of more asynchronous features such as auction houses, or who just wish to chat with their friends without having to actually team up with them. It's important to cater to these players as well as those who enjoy making the best of working as part of a team.
Bungie's upcoming MMO shooter Destiny looks set to follow in this mold -- though as part of an interview with GameInformer, Jason Jones explained that Destiny's endgame content would be much more rewarding and approachable if tackled as part of a group.
"We’re going to give you as many opportunities as we possibly can to expose you to other people, so that hopefully you’re drawn into some social experiences, because those are incredibly powerful and interesting, but we’re not going to force those on you," said Jones. “We describe it in a lot of ways as sloping the floor towards socialization, without putting a requirement on it."
Jones didn't explain specifically what Destiny's endgame activities will be like, but he noted that some of the most intense PvE content at high level would require cooperation between multiple players, but seemed somewhat hesitant to say that high-level players would be obliged to take on challenges as part of a group.
It would be in keeping with most other MMOs, however; even the most solo-friendly titles tend to become more group and raid-centric once you hit the level cap, with progression changing from being about earning XP to gearing up and getting the best equipment. Square Enix's recent Final Fantasy XIV, for example, features a main storyline questline that can be completed mostly solo with the exception of a few mandatory co-op dungeons -- but the finale of the story and endgame content beyond that is all reliant on playing with others. It sounds as if Destiny will be in a somewhat similar situation.
Destiny is out on September 9 next year. Does the news that you'll probably have to play nice with others at high level change your opinion of the game?