[fadetree]

Well, while I wouldn't compare classic raiding to chess, anytime you get a large group of people together you're going to have some difficulty just keeping all the cats herded and the idiots pointed in the right direction. With a group of seasoned raiders though, its pretty easy.
Velious got a little harder, if I recall, with some adds and things that you needed to handle and react to. Still not super hard but not ridiculously easy either. 'Hard' is always difficult to quantify, anyways. Knowing a big list of requirements like mob kill order, etc., is hard at first but stops being hard the second its written down and memorized. A long encounter always raises the probability of someone screwing up at some point, but a lot of people don't seem to think that counts as 'hard'.