While measuring the player "skills" and "effort" is usually easy, adding some "less objective" statistics can give the player supplementary goals, especially in a MUD/RPG context.
What I mean is that apart from counting how many orcs were killed, and gems collected, it would be interesting to have something along the line of the traditional Good/Evil, Lawful/Chaotic ranking of paper-based RPG, to add "dimension" to the game.
But computers cannot differentiate good/evil effectively (nor can humans in many cases), and if you have a set of "laws" which are precise enough that you can tell exactly when the player breaks them, then it generally makes more sense to actually prevent them from doing that action in the first place.
One example could be the creation/destruction axis (if players are at all allowed to create/build things), possibly in the form of the general effect of the player actions on "ecology".
So what else is there left that can be effectively measured and would provide a sense of "moral" for the player? The more axis I have to measure, the more goals the player can have, and therefore the longer the game can last. This also gives the players more ways of "differentiating" themselves among hordes of other players of the same "class" and similar "kit".