When you first get in there, things start out fairly easy-- Ramparts and Blood Furnace bosses have their quirks (the last guy in Blood Furnace plays out like a mini-Geddon with the AoE, but he gives you so much warning, it's no real trouble to get away in time), but the good stuff starts coming later. In Slave Pens, there are trash mobs (not bosses) that lay down mind control, and they come with a whole crowd of people. There are 5 man bosses in Auchindoin that do some nasty AoE and bosses that require DPS control (the stuff that Dives yells about). Durnholde and Black Morass in CoT are two tough instances that grab on to you and don't let go until you're all dead or the instance is over. And then, just when you think you've broken everything Blizzard has to offer, you try Heroic mode, and the difficulty ramps right back up again (perhaps in disproportion to the gear offered, but that's another story).
WoW is not a hard game by any means-- that's why so many people like it so much. But especially now, when almost no one has picked up the top endgame gear, more than a few groups are actually finding a serious challenge. I'm having a lot of fun working through it (wiping isn't that fun, but winning after the wipe is), but players who were just messing around in PUGs before are being forced to learn their stuff, or wipe out. It's a whole new endgame out there.