In my opinion, MMOs should be a virtual world as much as they should be merely a game. And bringing life to a world requires that they immitate life in certain respects, and I think not being able to jump ruins the sense of a well-rounded world by limiting the degrees of freedom with respect to our characters' movements. In real life I can jump to my heart's content, and yet I'm supposed to beleive I cannot do so in a game where I fight dragons?