Meh, Diablo 2 wasn't either yet compared to WOW it may as well have been: other than raids, WOW all boils down to 4/5-player-or-less encounters (heck, most of their raids are ten... nowadays...) or solo'ing quests. Diablo 2/3 have persistent characters, trading, etc. in them, so... it may as well be with how single-player WOW is in general except for the economy (which wouldn't be a big deal in Diablo 2/3, just advertise in the general chat lobbies for a trade). WOW's truly a "singleplayer" type MMO, where you don't really see or interact with those you do see, much with those around you: they may as well be NPC's.
Maybe that's why you don't like WoW? You anti-social?
If you engage the community (outside barrens/trade chat), you'll see it's much much more than a single player environment.
They tried to balance WoW for the people who like to spend some time alone and do stuff on their own (single to group quest balancing, achievements, dailies, etc) to people who want to raid or PvP in groups (battlegrounds, dungeons, raids).
There is a huge mix of things to do in WoW, that's why it's lasted so long.