NaughtyMuffin has three things that every good social MU* needs: it's relaxed, diverse, and fun.

Now, you'll note that I'm staff there, so perhaps I'm biased; but I was a player for over a year before I became staff.

It's relaxed; the staff there's tolerant of everything except being a jerk, and the playerbase, as a result, has few jerks. There's an actual policy about how you should relax and have fun playing - entitled News Pretendy Fun Time Games.

It's diverse; there's players and characters of every possible race, gender, and sexual orientation. I've heard the common complaint - that heterosexual males are ignored - but my male character hasn't experienced it, getting plenty of attention from females.

It's fun; in addition to News Pretendy, it has a theme (involving Half Life 2-style conquest by Puchuus and Raving Rabbids) that you can ignore at will, but which provides a platform for happy silliness.