Perhaps all MUDs did this, but my MUD of choice (Revenge of the Jedi RotJ) tallied your total playing time in your player stats.
The way I got started was that a few friends from school were playing and talking about it. It sounded dumb at the time (no pictures + typing) and I decided I would start playing just to kill them in the game.
Little did I know that I would need to spend a lot of time to improve my level, acquire skills, and get better equipment. In addition I didn't know that PKs were mostly frowned upon.