I think you should be looking into action games or something, difficulty really isn't a focus of MMOs. Or RPGs in general really.
In an MMO what people describe as "difficulty" is usually just bad design. The only things that are "hard" in WoW relate to RNG or ridiculously long grinds (not many examples of either of these left in the game because everyone hates that kind of thing). That's not hard, just shitty.
---------- Post added 2013-01-10 at 12:30 AM ----------
I agree with you, but heroic raiding is only difficult in a progression sense because again, you're trying to beat content early ie while still undergeared.
The main ACTUAL difficulty of raiding is getting all the right people online at the right time with the right mix of classes in the right gear and no lag spikes. And then coordinating them, communicating effectively with each other, avoiding guild drama... which is INCREDIBLY UNBELIEVABLY HARD, but not really in a gameplay sense. I reckon if you could somehow control 10 players at once all by yourself, a full heroic clear would actually be pretty easy. It's teamwork that's the challenge.
You won't find anything current that's harder than WoW's end game. EVE is a lot more complex, but EVE really should be it's own genre, it's nothing like any other MMO out there. Everquest 1 has a really small playerbase but it's content is more difficult, or it was last I checked. Everquest 2 has some difficult content and some really easy content. Final Fantasy XI (I think it's XI that wasn't the complete failure ya?) had some really difficult content as well. The thing is that those games tend to require a much larger time commitment than WoW as well.
There are probably some PvP MMOs out there that are harsh and unforgiving, but I can't think of any mainstream examples. Any of the big budget MMO releases tend to suffer from underestimating their playerbase and releasing undertuned content that's cleared almost as soon as it's released and leaves players extremely bored within a week or two of the first batch of people hitting level cap, which is why most of them go Free to Play within the first year and gone forever in the second.