Everyone is entitled to their opinion, their opinion doesn't represent the facts...
Anyway, it's not the length of the game that is most important IMO, and the story telling in D3 is awesome.
D3 was too short? too few ACTS that is? say that to the players who cleared Diablo II in 5 hours (ALL ACTS)
And people play it because they love hack'n'slash and smashing through waves of demons etc.
I love playing throught the same content, i'm having a great time smashing through everything with my barb.
And ofc there's the fact that we don't have any offline mode, but with an offline mode there wouldn't be any Auction House, and if you think about it, it could take many months to aquire the appropriate gear to clear the game on Inferno mode.(for the people whining about gear is too quick/easy to aquire)
The game haven't been out for very long.
And there are yet more content to come, like PvP and future expansions and fixes.
The game haven't failed at all, not by a long shot.