I've played off and on since day 1 of diablo 3. I started out as a barb and it was painfully hard, I then switched to a demon hunter and to be honest it wasn't much easier (I switched after they nerfed smoke screen)
The game is too much of a job, for whatever reason you want to list. It's a chore to a lot of people and not an enjoyable leisurely experience. I do not have 1000+ hours to put into this game and if I did I wouldn't want to drain all of my free time into one game. I don't have anywhere close to that amount of time.
Having said that, D3 is starting to come around. I do not want everything "handed" to me or the game made so easy that everyone can be the best with no effort. What I do want is to be able to make reasonable progress for the time put in. That of course varies by what your idea of "reasonable progress for time put in" is. I don't think I should have to farm for hundreds if not thousands of hours just to be decent.
A few fundamental flaws in the game:
1) The botters run wild and blizzard does nothing to stop them. There are botters who are 100% no questions asked botting. Accounts that are logged on and playing for 23+ hours out of a day each and every day are botters. You don't need to do anything at all other then swing the ban hammer to get rid of those botters. The causal botters would be tougher. But if you can't remove the botters then what you can do is reduce their effectiveness. Reduce the need to use the AH, reduce the need to have tons of gold.
2) Reinstate the ladder. This is 100% a must. I have no idea why they went away from it but we NEED the ladder system back. It levels the playing field each and every year (or six months or whatever) and allows players to start over fresh even with each other. It gives players a lot of new goals (even though they are the same goals over and over) such as server first sixty, server first paragon 100, make the top 20 barb page.... whatever. But players won't care as much about the super rich and the bots if everyone starts over at the same page every few months to a year.
3) Try to make the game less linear. An idea i have is similar to the OP's... like in WoW have you have class quests how about having a random set of quests specific to each class? The reward for completing all the quests would be a "good" peice of gear. For barbs say a ring with 150 str, IAS, CC% and maybe a random roll between LOH, All Res, or movement speed. Or something... something that you KNOW will be a decent ring for all but the super rich. Make the quests get rarer as time goes on, but not something stupidly hard.
4) Remove the 100% utter junk drops from the game. There are some drops that no one will ever want to use ever unless they are entering a "most useless gear ever" contest. Cap it so that there aren't mountains of trash dropping all the time. Players need a better chance at finding upgrades. A much better chance. Again don't hand us everything on a silver platter, but don't insult us.....
5) Personally I would make it 8 players in a game. Diablo 2 was a social experience... D3 not so much. Usually in a 4 player game at least one person is AFK or off doing whatever/wherever and isn't talking to the rest of the group.
Blizzard is doing better, but much still needs to improve. Right now there is nothing to play for except to increase your E-Peen score. The game is so linear it's painful. But there is potential.
Maybe even allow players an option to play under "classic" difficulty. Who like to get raped by the inferno of old. I didn't but I know some people did.