My inventory is starting to run full, and then I accidentally click on the "Cracked Useless Dagger of Bluntness". I open up my inventory and see the dagger. Is the only way to drop it to click and drag it off the inventory screen? Or is there some keyboard/mouse combination that will drop the item without me needing to move my mouse half way across the screen?
It is easiest to not drop items and instead use the town portal you are granted relatively quickly in the game. Whenever* you're full, you press T, sell non-magic items, salvage magic items, update your stash as needed and jump right back in action.
*Doesn't work in boss levels until after the boss dies.