I need to open every container and take everything with a good price/weight ratio. I will haul a dozen spoons out of a dungeon if they have a good ratio. This applies even when I more money than I'll ever need. I rarely ever invest in merchant or speech skills. I don't need the gold, I just need to sell stuff.
On a related note, when I inevitably dump this stuff in a chest somewhere, I always make a save that I don't overwrite. Just on the off chance the items suddenly disappear in ~20 hours of gameplay or something. I don't think I'd ever actually go back that far, but I keep the save anyway.
I always save potions/scrolls or whatever items that temporarily boost my abilities, even if it means dying a lot. Somewhere in my head I always think that I shouldn't waste them because I might need them later.
Then during the final boss fight of the game, with my inventory full of those buff items, I still refuse to use them.
I hate using consumable items. They usually have a good price/weight ratio, so I can't part with them. I just started a new Fallout 3 playthrough with Wanderers Edition mod, which removes the healing from water and food, so you need stimpacks to keep yourself alive. The problem is, they have a fantastic 350 caps/pound ratio! I've been running around the Wasteland for the last two hours at ~30% health because I can't bring myself to use them, and I'll occasionally reload an old save if I take too much damage in a fight because I just can't bring myself to use a stimpack instead. I have a couple dozen in my inventory.