AstroMUD is a really nice CircleMUD-based MUD with a lot of loyal players that have been there for a long time. Many original zones, and a partially revamped codebase.

Multiclassing (up to 15 distinct classes), 20 different races, and deity branding (with special powers attributed to each brand). 40 different skills, 95 different spells, and some additional spells/skills that are innate and as such not listed on the official lists (see inventory, etc.)

Other bonusing available: clan tokens, tokens (and daily quests), hero and valor emblems, puzzle pieces (need to collect them all, and then combine to get nice equipment)

Lots of additional social commands. Limited PKILL mud (only specific areas of the mud are PKILLable, unless you're a clan warrior).

Be prepared to die to get the really nice equipment. It's not easy to get. The nicer the equipment, the more difficult it is to get. Team kills are encouraged.