I used to play an LPMud for about five years. It was heavily skill based in that basically the whole game's character advancement was built around the trees of fighting, magic, faith, covert and other.
Every action in the game, whether it be a ritual (faith/priest spells), spell, special attack or covert action relied on skillchecks on a multitude of skills.
For example a special attack while you were in combat might use fighting.melee.sword to determine damage and landing, fighting.special.weapon to determine success rate to prepare, fighting.special.tactics to determine how much effort it took, and fighting.points to determine whether you had enough spare fighting 'effort'/'stamina' to execute the move.
Ambush would you covert.hiding.person, covert.items.weapons, covert.stealth.outside/inside, covert.points and fighting.melee.daggers, etc.
I'm a bit tired of MMOs where everything is simply based around your character level and the trained level of the spell/ability you are using. Apparently Ultima used to have a similar system to what I was familiar with, but I have not seen it in any mainstream MMO thus far. Everything in terms of skillchecks seems to be all very basic and one dimensional.