Monster power- introduces very hard difficulty, but you dont have to play on anything higher than mp0, where might I add you get the most exp and items (in act 3 anyway and the exp rewards per mp level are being heavily buffed in 1.07, so thats a plus at least.).
Previous to any patch, inferno was way harder than even mp10 today, in its own unforgiving way. But players couldnt simply reduce the difficulty to faceroll farm end game gear more effeciently than ever, causing devestation to the economy.
It was a good try, and I liked the idea at first. But its apparent now that the system removes much of the reward of advancement away from the game. (actually 1.03 killed inferno difficulty first, but monster power compounded it.)
Battle net 0.2- Sorry blizz, love you guys and support D3, But the new "battle.net" is such a downgrade to diablo 2's battle.net. This part imo took many many steps backwards and removed some of the best tools to players in Diablo (game names, custom chat channels, /f m, player avatars in channel, etc.).
Lack of end game content- Previous to patch 1.03, The end game was inferno. It was very highly unlikely that you would enter inferno for the first time and actualy complete act 1 without seriously overhauling/upgrading your gear and build. Once you finaly did find yourself in act 2, you faced such a ramp up in difficulty it was brutal, but very rewarding once you were able to progress through it. Once you finaly did get through act 2, you meet inferno belial. LOL. Took me and all my freinds WEEKS to finaly down belial, It was a true achievement. Freinds that were not in my game and saw my achievement for killing him immediately whisper me with congradulations and disbeliefe over my victory. Act 3... lol. Act 4 really actualy wasnt any harder than A3 though, but still, they were both MUCH harder than A2. (except for azmo, he was always easy.)
Now you just set mp0 and the faceroll continues from hell into inferno, turning infenro into just another difficulty instead of its intent as an end-game challenge that only dedicated players could accomplish.
The infernal machine event did add to endgame, but not nearly enough. We have, act 3 mf/xp runs (they are both the same run btw lol.) Or craft hellfire rings. Yay! 2 end game things to do.
Now we'll have:
act 3 mf/xp
account bound crafts
Woot 4 endgame activities, oh the variety.
Its my oppinion that these are the 3 biggest reasons that people are leaving.
edit: Dont take this post the wrong way, I dont hate d3, its one of my favorite games. But I can look at my own enjoyments in an unbiased manner and admit the games major faults.