I think the issue with Diablo 3 and people's dissatisfaction with it is one of unrealistic expectations.
We've gotten used to a certain type of gameplay over the last years from World of Warcraft, and we somehow, perhaps subconsciously, expected that Diablo would deliver something very similar.
Instead, Diablo III, as far as I can tell, has stayed very true to the heart and soul of Diablo II. Of course, some changes have been made along the way (personally, I find most of them to be improvements), but it's essentially the same game.
I'm not playing Diablo III anymore because I'm too busy with Mists of Pandaria testing, but on my way through Inferno I never found it necessary to use the auction house (the real money one had not yet been implemented). So, that is one concern I absolutely don't share with people, namely that the RMAH is somehow ruining the game.
I think Diablo III will have very good longevity and I believe there is a great number of people, part of the "unspoken masses", who are still happily playing the game and will do so for a very long time.
It's just jaded veterans that had a nostalgic feeling about Diablo II, and we thought we could get that back despite the fact that we were mostly in our teens when we played Diablo II and things are now very different for most us.
And of course, some players who never played Diablo II and were used to the style of World of Warcraft might be disappointed as well.