Apologies, I know it says #1, but I couldn't resist. It is also a compilation of other people's ideas, so I take no credit for it, but I think it is worthwhile repeating.
10) Increase drop rate of lower level sets and legendaries only. Encourages 2 level 60 chars achievements and keeps a fun pace. Finding legendaries once a week is not fun.
9) Tighten item stats. Level 63 1H AS, Crit Dmg, Crit Chance, etc and 200 DPS is not fun to find.
8) Increase Movement speed. Boots with 12% speed are mandatory, why? You increased the pick up radius. Why not the speed too?
7) Decrease gear dependency. In D2, even blues were useful. Why? Because character with high enough level could actually survive. Yes, I know the AH is important for cash flow, but at least make the characters a little stronger. A Nephelem (sp?) is godly solemnly because of its gear?
6) Increase monster density on Acts 1 and 2 by 200% for nightmare and up. Increase to 50% for Acts 3 and 4. I know MP increases difficulty, but I am speaking of hack-n-slash god-like power, kill by the hundreds with 1 sweep fun. It is supposed to feel this way, right?
5) Lock item and skill slot option. During frantic fights and raging selling, mouse missteps occur.
4) Alternate Skill set or Build set save feature.
3) More gem variety. What happened to diamonds, skulls and sapphires? On a side note, emeralds are still #1 even with the ruby buff. How about AR or IAS? Make sockets an important part of the game.
2) Option to turn off the pop-up conversations. God, they were really cool first few times, now, it is just annoying!!
1) Finally - ID all feature!! Yes, efficient farming has become the priority.
I know you guys at Blizz need to save some features for the expansion, but I am quite sure that your creative minds will come up with even cooler stuff.