Most "reaver solo'ing" is done by abusing AI redundancies and usually done by individuals who are "epic'd out". You rarely hear about joe shmoe solo'ing the reaver.
Also, I've yet to see anyone claim to have solo'd the reaver on anything but normal. And if someone out there has the cajones to say that they have done so, well then I demand a screenie. I take no man's word on the forums (save a select few). That alone answers your question regarding "difficulty". While yes, this is a group oriented game, there aren't many dungeons out there that can't be solo'd at the very least on normal. By the time you're level 10 you should have well enough gear to be able to solo every mission to elite, lvl 7 and down. Again, minus a select few exceptions. Namely those requiring more then one person to advance (switches and the like).
I think the one thing that the game needs more than enything is the ommision of the kill counter. People would be less apt to run off on their own w/o that oh-so-important kill count prodding them on. That, in and of itself, I think would add a good level of difficulty to the game.
... for some players at least.