I've been hearing a lot of that lately, even from hardcore Diablo fans. I still have not put any time into it at all so I can't say how much truth there is, but it sounds like people are getting bored quick due to the lack of any character building. It's become so loot-centric that the only way to succeed is by farming for new items, which many people just skip and buy their gear. So basically everyone has identical characters and the only reason to keep coming back is to farm some more. Diablo 2 had the options of making builds, which were just as important as loot. I can acknowledge that there were only a couple truly useful builds for each character but the variety was nice, and you could get away with having mediocre gear as long as you knew where to place your points. I can say for PVP in Diablo 2 the builds were VERY important, especially if you were targeting a certain character class, you could have a whole roster of Barbarians each built to kill a certain class effectively (speed barbs, resistance barbs, frenzy barbs etc etc). I guess that could be possible with gear to replace the modifiers but it seems kind of empty like that.
I assume the community is still doing very well but the fact that most of my friends have already stopped playing (people that played D1 and D2 for years after release) kind of bugs me.