Actiblizz screwed up here. Big time. Damage modifiers used to be in the skill trees, and your stat points. Sure there were a few + to skills items, but the bulk of your damage came from where YOU decided to place skills and stats.
Fast forward to d3, and we find damage modifiers on most gear slots. This means 99% of what we find will be useless because the desirable affixes wont be there. Had the bulk of damage modifiers been in skills and stats, players would have much more flexibility in gear, and the gap between junk and good would be smaller. This means more usable items, and makes pay to win much less appealing.
Now comes the weapon failure. It used to be that physical damage classes were much more dependent on weapons than casters, as it should be. Magic damage got its power from the level of the skill, not the size of the weapon equipped. Now everything is mindlessly based on x% of weapon damage.
How the RMAH plays into it all. By taking the damage modifiers out of the skill/stat trees and putting them onto gear slots with rng determining the usability of the item, players are steered towards the auction houses, where they will likely end up needing to buy gold to buy gear, or simply buy items straight from the RMAH.
Yes, we found a lot of junk in d2. However, you were more likely to find a usable item. Heres why. Assuming a 6 affix roll in d2, the probability was much higher that your item was useful because there were less possible affixes to roll. By putting damage modifiers into the list of possible affixes, the chances of finding a useful item drop. This is how d3 is tuned to steer people to the RMAH.