In my opinion, the primary drawback is exact knowledge of stats and their relation to increases in ____ value. This game, and others that allow this (most every MMO nowadays) just boils down to a mathematical formula to decide which gear is best, and if you're not getting that, you're not having fun. I think some of the tech advances (and/or design advances) we've made in certain genres has severely hampered the overall fun value of the games.
Off the top of my head, both Diablo 2 and Everquest (early) were much more fun than their counterpartsfor mewhen it came to gear acquisition and enjoyment of that whole aspect. It's MUCH more entertaining to be able to just try shit out because it looks cool, or is fun to play that style, or whatever, when things are a lot more nebulous and all you have is a general idea of what is good. In D3, if it drops your DPS by 3%, odds are you aren't going to equip it, even if it looks cooler, or allows you to cast a different spell for some more varied fun, because our brains are like "No, but wait, this is worse, the game says so right there."
That's my take at least. The engine is great, and it feels fun to play, but the item grind is absolute shit, and I think this is a huge culprit personally.