I've been on a half dozen or so MU* before, but OtherSpace is fast becoming my favorite. There always seems to be something going on somewhere at any time of day or night. If there's nothing going on where you are, there's usually some other place you can get to where RP is active.
OtherSpace has an original theme with diverse races and options that allow for a vast variety of characters to be made to suit most any person. Just as the theme is original, player originality is encouraged to. This results in one of the most high quality player bases I've yet seen, yet without the sense of elitism that makes some high quality player bases unapproachable for newbies. Both players and admin are generally very helpful and fun to be around.
Another thing that keeps life moving at OtherSpace is the constant presence of a "Story Arc". Each of these master plots is carefully woven into the continuing story, and designed in such a way that the players can get into and effect Arc events, whether on purpose or by accident. I myself have recently experienced the latter, having stuck my nose in (gotta stop doing that) on an argument that has eventually lead to political intrigue, press release wars, and a bounty on my head that's crimping my social calendar. But all of it has been great fun.
Besides the well-crafted and lively world, OtherSpace offers a well-designed website with plenty of information for new and old players alike. An easy to use web-based discussion board provides a forum for players to make suggestions and share roleplaying tips, and even ask theme questions. An optional mailing list can keep you up to date on the latest major happenings.
OtherSpace provides a well-rounded background against which to pursue one of many possible lifestyles and affect anything from your local tavern crowd to the fate of millions. I highly recommend it.