As far as MUD themes are concurred they can be anything from being based on books and can be in a science fiction or fantasy theme.  This means that you can experience anything from a romance to a space combat.  They can also follow book themes like the Dragon riders of Pern.  Others have been known to follow anime such as Robotech.  One such MUD that I play is fantasy/medieval know as Rivers of Mist.

On the MUDs you can meet people from anywhere in the world and form friendships with these people.  You may learn things that you never have known before you met them there and you could tell them things that you have never known before.  Basically, you can meet people and learn things that you never would have had the possibly to do before or ever.

In MUDs you can play character that represent you and your personality, or play a character that interests you.  The character race, sex and class are purely your to choose for the character.  The race is what your character is; it can range from it be human to being elf, orc, or some space alien.  The class is what your character does for a living.  This class can range from warrior to a pilot.

As you play the game you can gain levels, experience, and if its part of the MUD you can learn to cast magic.  The more you play the character the more likely you are going to learn skills in the class that you have selected for your character.  Just as there are themes in the MUDs there may be a story line that your character may affect.  Or there is an adventure that you can participate in.

In this I have said what I believe why people should play MUDs and why they are good for people to get together and have fun.  What type of fun you have and if other will allow it is up to you and them.  I wish you happy hunting and have a happy roleplaying.  www.mudconnect.com and www.wco.com/~rika/lo/ are my favorites.