I'm on my first playthrough, normal difficulty, but I don't see how Blizzard keeps up long-term player interest in the game. D2 was about customizing and loot, neither of which matter in D3.
By loot not mattering, I mean that the efficiency of an auction house and lack of means of item destruction (bind on equip mainly) ensure that most items will rapidly drop in price. The only items that will be hard to attain will be at the very top, where the hardcore players lie.
I'm on my first character, level 16, and he's already twinked out with rares on most equipment. I bought everything for cheap on the AH and I've already started dumping my first set back on the AH. I don't see this changing any time soon. There's no excitement in a rare drop when they're so easily obtained.
Speaking of the auction house, I don't understand, with their experience in WOW, how they can make such a crappy AH. I'm not talking about the limits on selling stuff, because with their financial interest I can understand placing the limits. I'm talking about basics like being able to sort by more than dps/ac and bid price. You can't even sort them by time, so I have to sort them by buyout. There's too many items and characters level too fast to make sorting through random-ending auctions without buyouts worthwhile.