Yes, on the item colelction side, there's little that'll keep you coming back for more. Definitely not anywhere near the depth in Diablo 1 or 2. Still, I found it almost a "cute" version of a Diablo game, one I had fun with, and that I plan to go back through with the other characters as well. It kept me entertained for a few hours, but I don't picture myself playing it much once the other characters are "maxed out" unless friends also get it.
Can't weigh in on the online portion, sorry. Haven't been able to play with anyone yet, though I imagine something like dropping a high-level item for them to pick up could help unbalance the game in their favor or help them towards achievements (like the one for having 500,000 gold).
Looking For Elliott's Portrait
in Fable 3!
Message me if you're organizing something in multiplayer for Red Dead Redemption!