The problem that I see with this, even amongst those this style of gameplay appeals to, is to support a fun and balanced gameplay experience in a permadeath environment, the game must be built from the ground up with this possibility in mind.

Take WoW, for example. (I play WoW, so it's the best existing mmo I can relate to.) It was most definately not built with the vague possibility of a permadeath environment in mind. Characters -will- die. I can think of a boss or two where a character death or two is not just likely, it's -guaranteed-. You couldn't even begin to maintain a raid that way, and learning new boss encounters would be laughably impossible. Even apart from endgame, there are plenty, -plenty- of times that just plain bad luck will off you regardless of how "uber" you are. Quite a few mmo's have a calculated amount of likely death/corpse runs/etc. as a time/money sink. To make it vaguely fair or fun to do a hardcore mode, adding permadeath to an existing game would involve a ground-up mechanics retooling.

Now, I acknowledge I don't know Dofus from DOMO, so some of these examples may not apply. One problem that will remain a universal difficulty for any MMO considering a permadeath environment, however, is the cold hard limits and fallibility of the technology.

Lag happens. Disconnects happen. In the average mmo situation, a gigantic lag spike means some annoyed grumbling, possibly a corpse run, and probably an xp or gold sink. In a permadeath environment, it means game over. Start again. By no fault of your own, your character is gone.

Imagine grinding on things far beneath you for weeks, slowly building your levels high enough to feel you can safely take on that next big challenge. Your bags are brimming with consumeables you've spent countless hours farming for, your gear dripping with every last upgrade you could squeeze out at your current level. This is it. You take a deep breath, and rush the quest kill...

And the game suddenly freezes. You frantically hammer on your combat macros, but to no avail. You wait...finally with a rush of delayed sound effects crashing back to life, the lag ends...with you dead.

How many times would that be fun?

In this scenario, any server performance one whit below sub-optimal would elicit outcries from every player on it. People would watch their pings like crack-riddled hawks and the server would stagnate as noone would group with anyone they didn't have a long history with and absolute confidence in, and even then noone would set foot outside the safe zones without a ping remaining firmly in double digits. The gm staff would be inundated with complaints about lag death, or death due to other bugs/glitches. This is, to my guess, the biggest reason that most MMO's will not incorporate a permadeath/"hardcore" server despite requests, because of the huge amount of work this would generate vs. business generated.