Two good threads covered this recently:
http://musoapbox.net/topic/1063/how-does-a-mu-become-successful?page=1
And
http://musoapbox.net/topic/1043/meta-vs-prp-vs-planning-vs-impromptu?page=1
The later talks about staff provided things that make it fun for players ... which boils down to having something to do.
That said, I'll cast my stone with the 'whatever you have fun doing' lot.
If you have fun with 8 people on your own place and they all have fun, running plots for each other or whatever happens in the theme on a regular basis to create story and RP, then you have succeeded. If you want to run things, then maybe having 60 unique individuals with 80-120 unique chars amongst them, all doing things while you throw out meta plot to the various circles and everyone is having fun, then you've succeeded.
Remember the nearly golden rule, you can't please everyone all the time. The places that get the biggest gripes are usually the 'bigger' places, generally having 30-40+ chars on during peak times (even if its 20 people with lots of alts). The gripes always seem to come when the fun of an individual doesn't jive with the fun of others somehow. Its rare to see a place with less than 10 chars active on a daily basis get a complaint here (if ever), and usually those places are just fine with the 10 regulars that keep on telling stories.