Reading this story and many of the responses I'm surprised that there aren't more akin to this. While some of the MMOs have been addictive to me (no more, children are much more fun) every game I've played that has given me the most enjoyment has been for reasons that you describe.
MUDs, Doom, Duke Nukem, Quake, Unreal, gobs of RPG type games and, lastly, MMOs provided me ample opportunity tweak and see what I could make them do. In some cases it was directly and easily modified. Other cases required external tools, but in every case I was interested in seeing how far I could push the mechanics.
Playing the games are fun. Pushing them to their limits are what I enjoy. This is pretty much true for me in most aspects of my life, though.