1. You never find items that make you say "I want to develop my character to use this!".
Firstly, you never find items for your class that you can't use. Unlike Diablo 2, items only have class and level restrictions, no attribute minimums. This is because Diablo 3 autolevels your attributes when your character advances and doesn't give you an option on how to spend your points. So unlike Diablo 2, it's impossible to find an item that you can't use yet, but that you will be able to use if you spend your next two or three levels putting points into a specific attribute.
Secondly, there is no character development. When you level-up you don't get to spend points to improve anything, you simply get access to new and different skills. The new skills aren't "better" than the old skills, just different. There is no sense of progression whatsoever, and no sense that you're tailoring your character according to your desires. There are no trade-offs. This means that the items you find also have no trade-offs, which makes them very uninteresting. Except for your very first choice in the game when you decide what class to play, you never get to make any choices that affect how your character develops. Every item you find either improves your primary stat and vitality (better, use it), or it doesn't (worse, sell it). There are no decisions.
2. The auction house. It's much easier to buy good items than to find them. You can buy amazing items for very little gold, which takes all the excitement out of finding loot. Nothing you find will be better than what you can buy.