With a few exceptions, MMOs operate on a fixed level of difficulty for all of their players. Instead of assigning blanket difficulty client-side, the game world portions difficulty into areas, usually according to level or activity. Some games have instances with adjustable difficulty levels, but past that what you get is also what I get.
This might be changing. A very fringe but dedicated group of players have championed such ideas as elective ironman and permadeath modes for their MMOs, and at least one studio is responding positively to that desire. Would you choose to make your MMO experience harder than everyone else in exchange for nothing more than a bigger challenge and a more "realistic" experience?