OK, wanted to play for a while before making any suggestions.
1. Dont have the game move stuff around in inventories. I have my potions divided between my characters to avoid encumbrance. Every time I interact with a merchant it dumps all the potions back into one inventory.
2. Dont have the game reset the minimap. Every time I repair something it turns the minimap off again.
3. Dont have the bottom levels of battlefields empty - if there wont be a quest completing npc down there put something down there to make it worth while.
4. Have the alt key display all possibilities including books, barrels etc. The game ate a quest item (book for Crazy Barnabus) but I didnt know if it was gone or it had dropped when I moved it to the other character. Went back and spent 20 minutes moving the mouse all over the area because books wouldnt show up by hitting the alt key (in truth it was truly gone - bad bug).
5. Weapons and equipment need to be upgraded. Currently I am at the end of Act 3 (well stuck in a battlefield in act 3 waiting for a patch to let me out,but ready to go down in the temple). Still using equipment gained in Act 2 because I havent found anything better as loot or for sale (well, found a great long sword but after I identified it, it disappered, but even that was in act 2). Also along this line the "uniques" just arent unique enough, I have found them and havent found a single one that was better than non-unique stuff I had picked up, and I cant even sell them most of the time as they dont show up in the trade screen, have just dropped them all over the landscape.