I suppose I can say little about 3 Kingdoms that hasn't already been said.
As has been pointed out, 3k has a massive (and expanding) mud environment, which means you will never run out of places to explore. 3k has a large variety of guilds, each with their own strengths and weaknesses, along with many unique powers for each guild. 3k has a very well developed newbie system, with a dedicated newbie area as well as a large group of players who have volunteered to be Newbie Helpers. 3k has a wonderful quest system, endless variety of character manipulation, PK and simPK, and too much more to list.
However, I feel the best part about 3k is the players. The atmosphere on 3k is very friendly, and a huge part of that is down to the players. There is constant banter on public lines, and the sense of humor of most players are well developed. The jokes range from bawdy to classy, and even a few classy bawdy jokes appear from time to time.
It's not hard to find someone to lend you a hand if you're in need, and you find yourself willing to give your own time and knowledge to help others. It's this spirit of community that makes 3k great.
In summary, I heartily suggest trying out 3k. There is simply no better.
Necromancer of the white material planes.