THINGS I WOULD LIKE TO SEE
I dont know about everyone else but what really bugged me was that 6 tiny gems or runes the size of a postage stamp took up as much carrying space as a full sized double handed battle-axe! I know realism is all to hell anyway and it was probably done to balance out the game and prevent the over-usage of charms yada yada yada.... but I much prefered the Fallout system which was weight oriented. It meant stronger characters could carry heavier equipment or wear more cumbersome armour and small items could be carried in far more useful quantities. I think it would work quite nicely for Diablo. It would prevent level one sorceress mules wandering about with 12 unique axes for trading, you would have to invest time into creating mules and so would be required to be a lot more critical of what you collect or at least to be a far more active trader rather than a hoarder. It would replace the strength requirements for armour as well and prevent the "I need one more point of strength before I can use this" scenario that I hate. You would be able to wear a slightly heavier armour but would have to downgrade another piece of armour to compensate or empty your backpack and be content to be unable to collect extra items you find that are above your weight limitations.
Also the closer you are to your weight limit the slower you would go, which is far more realistic. It would mean that whilst full plate would really slow a sorceress down it would have very little effect on a high level barbarian strong enough to carry the weight without penalty. Best of all you can carry gems and runes without it being to the exclusion of all else. For every character to be able to carry a pouch of 20-30 runes and/or gems would make them much more of a tradeable commodity and reduce the need for unnecessary muling.
Perhaps require sacrifices from players to progress through games, sacrificing prized posessions or character attributes to pass certain barrier quests. This would prevent base level characters from being easily rushed through the game as they may not have obtained the required sacrifices yet. Could be wrong though, I can see some ways in which that could be real annoying.
I would love to see Blizzard get a LOT more involved with the storyline this time! There is nothing better than a story with complexity and twists and turns that you dont expect. The story always seemed a very distant part of D2 in comparison to killing shit. Perhaps by including more interactivity actually IN the combat environment and including lots more quests both small and large that tell the story in a slower more intiguing way. I would rather have twenty very small cut-scenes spread over closer intervals keeping me interested in the story progression, 1 per act never seemed quite enough.
More variety of items! I know I'm gonna get shot down on this one but although there were LOADS of different item choices in D2 lets be honest, when you were doing you're character build for the barbarian it always boiled down to a 5 socket runeword on the best axe possible. All i'm saying is why not make ALL the different pieces of equipment more useful in different and unexpected ways. I must admit that is very vague but I would just like to see people building their characters in far more varied and versatile ways.
Mixed abilities. I would love to see a barbarian with a weak but tailored spellcaster ability or a sorceress that can utilise a limited necromantic or druidic streak to change the way her abilities operate. Perhaps I'm asking too much.
THINGS I HATE
As already mentioned by others, replacing potions after death.
The instant death curse of the Oblivion knights!
In Diablo1 the quests sometimes varied, you never were certain of where you would go next time. This was a major drawback of D2 in that the game was always exactly the same structure. Having to play the game 3 times in different difficulties I would have really appreciated a bit of variety in the experience.