I have thoroughly enjoyed many aspects about Diablo 3. There are other things about it that I have not been so fond of (always online, AH, low drop rates, cookie cutter builds, no content beyond level 60, and a shallow unengaging story). The expansion can fix many things, it could be a way to reintroduce things from D2:LoD that are much needed. Rune words were one of the best and most unique feature the LoD itemization system had to offer.
Add in a couple of new classes, like the necromancer, or totally blow us out of the water with something new and unimagined. How about a renegade Demon with his own agenda? Or a morally ambiguous angel trying to find his footing in a universe filled with conflict. Most of the characters overt I am the hero and must always be a goody-two-shoes lacked any rpg aspects what so ever.
As far as story, make it more interactive. Give our character some choices in game, how about instead of always killing the villains and being the cut and dry "lightsider", why not give us the choice to aid the villains, or claim power for ourselves and our own purposes. An RPG should have some sort of choice.
Better and more engaging quests, with actual quest rewards, lets face it 16000 xp and 700 gold for a quest on inferno is laughable. In d2:LoD certain quest givers would reward you with rare rings and rune words, in TL2 the same is true, minus the rune words.
Randomized dungeons and world map randomization and a much larger open world are a must. Give us reason to pour hours exploring the map and dungeon diving, so that we aren't seeing the exact same Field of Misery with 10 hostile mobs, which leads to my next suggestion.
Enemy density on many maps was seriously lacking and left many maps feeling empty with little to kill. Greatly increase the enemy density. D2 constanly had wave of nasties for you to burn through.
I know the game series is called Diablo, but seriously in D3 he was he/she was comical at best. Bring back Lord Baal, and Mephisto their screen time has been limited, and frankly they are much more interesting than the Lord of Terror. I like Azmodan as well and found myself rooting for him, but that is more personal preference :)
Again, I'm not going to bash, I have fun in d3, and want to continue having as much fun in this game as I had/have when I boot up d2:LoD. The game has the raw materials to be great, I would love to see it truly shine!
P.S. Apologies for the multitude of grammatical errors and typo's.