Since Borderlands 2 came out today it will be a while before I play D3. BL2 is an FPS, so it's not the same genre as D3, but the basics of the game are the same, i.e., do quests, find loot, make money, get better gear, do more damage.
The game has several advantages over D3 as far as gameplay.
1. No need to ID drops
2. Complex builds mean no cookie cutter characters
3. Can go between acts (worlds) in a single game
4. Can create custom game
5. Multiple active quests
D3 has these advantages over BL2
1. Stash is account wide
2. Loot drops for individual players (no fighting over it!)
3. Paragon levels
The reason why I find BL/BL2 more enjoyable than D3 is because it's more rewarding. No one expects every drop to be the ultimate drop, but, while there are plenty of garbage drops in BL, the amount of good drops is more numerous. In D3, you can farm an entire act and realistically not get a decent drop. That concept makes the game for me - give me something exciting for all the hours I've spent playing. It doesn't have to be godly, but it should at least be comparable to what I'm currently using.
Another problem that leads me away from D3 is the wall I've reached at A3. A1 and A2 were hard, but they were hard enough to actually WANT to rip through them. The transition from A2 to A3 goes from hard to impossible. Inferno should be hard. That's the way the game is structured. It's not that it's hard, it's that that the difficulty curve is more like a difficulty right angle. There's a Catch-22 with A3 - you really can't start farming A3 unless you have the good gear that drops in A3. Well...initially you CAN farm it but you'll either 1) mooch off of other player's abilities or 2) die alot... I mean... really... alot.
The AH is an option. The idea of an AH looks good on paper, but in reality it has become an overpriced, necessary part of getting ahead in the game. Sure, you don't have to use the AH. But you will definitely reach that big frustration wall if you don't. The AH should be an option. It is really nothing more than a way to diminish the frustration without actually playing the game.
BL is hard, but there are so many things to go after at any one time, you don't have to hit that wall of frustration. I'll keep my D3 character, I'll log in every now and then to make sure he's kept alive, and I'm in no way saying that D3 is a bad game. It's given me a few months of good gameplay. But when there's another game available, the boredom/frustration level makes it easy to leave.
Maybe the developers will understand that we play games for enjoyment, not frustration. I started playing D2 on the day it was released and it had lots of problems when it was first released. But it got better, especially with LoD. I suspect D3 will eventually get better, but Blizzard should recognize that they won't get back the number of people they'll lose while waiting for the game to get better. Couple that with the natural loss of players as the game ages. They are now in direct competition with new ways to keep us entertained.