“It’s more fun to be around people doing a somewhat fun activity than it is for me to be alone playing great content.”
And that just about sums up MMO player economics. No matter how spectacularly designed a city, area, or entire MMO is… no one will want to play it alone. The plethora of cries for “solo” content says more about people wanting to play alone in the general vicinity other people, rather than truly alone. All other features aside, the ultimate deciding factor in long-term success is both the strength and size of the community which supports it.