So how did Diablo 2 have "years of content?" And did it really or was it just a niche group of hardcore players who just played it over and over? It seems like Diablo 3 has a decent amount of content but after you've beat it on inferno (which some have already done, 3 weeks after release) you're just farming equipment to sell on the AH?
People played multiple characters and since you couldn't respec your talents or redistribute your attribute points - not only would you play with all the classes but within each class you'd have to reroll if you wanted to try a different spec. Not to mention, because of this, the first 30 or so hours
anyone played was pretty much moot because players did stuff like put tons of points into Energy and not enough into Vitality (so when they hit late Nightmare/Hell they were screwed unless they rerolled since they couldn't redistribute). As for items, people liked to min/max, they'd completely finish the game and kill Diablo on Hell but they'd continue hunting for items just so they could make their damage numbers bigger and bigger for the hell of it (epeen within the community) - I think that's definitely still going to happen in Diablo III. We'll eventually all kill Diablo on Inferno but we'll keep playing that same character and try to make them better anyways.
Honestly, I think D3 has even longer legs. I don't know if people will play it as long since the video gaming environment is very different than it was 12 years ago but it definitely has enough content to keep people playing for at least as long. Sure, you don't have to reroll each class multiple times to try different specs anymore, but it takes much much longer to "finish" the game (beat Diablo on the highest difficulty) than it did in D2.