Ok, I read the rest of the reviews and thought I'd toss in overall impression and comment/add to what else was said.
I've been playing on 3K just short of a decade, and have played a variety of other muds as well. Peronally I prefer 3K for the large player base, the gigantic size (over 96K rooms, and I've only seen about 1/4 of it in nearly 10 years), the variety of themes, and the social interaction.
Yes, there are the problems with the occasion jerk, player or wiz, just as as there are in real life. Occasionally the powers that be seem to take things to an extreme, but since I'm not privy to everything they are, I couldn't honestly verify or denounce their justification. Overall though, I'd say that if you're there to have fun and play and not be an idiot, then you're likely to never have problems.
While questing is no longer enforced (cept the newbie academy and various guild quests), there are benefits to questing. Through questing you can achieve numerous High Mortal ranks which afford bonus powers. So in addition to exploring, fighting, and solving puzzles, there is ultimately even more incentive to quest.
3K players are only allowed a single character to start; however, you can have a second character at level 50 and a third character once your second character reaches level 50. If you're the type of player who prefers to have a ton of different characters, you may find yourself getting wiped for having illegal characters. However, you can leave a guild and join another at any time after level 5, so your character is in no way limited to a single permanent path.
Finally, 3K isn't heavily into formal roleplaying. There are a number of guilds with stricter guildlines (such as the Breed who's guild and powers are masked in secrecy, even from other members or the Knight with their Code of Honor which all their members must adhere to), but for the most part the actual roleplaying aspect is fairly relax. If you prefer to be more formal in speech and action then that's up to you, but don't be surprised if people don't follow the same guidelines you set up for yourself.
All in all, 3K is a blast. Although there are some things I'd be interested in seeing in it (ranged combat, for example), it's a very thorough MUD as it is with a friendly atmosphere, large player base, and tons of opportunities to just relax and have fun.
Kryllith (aka Vamore/Althalus)