I keep every single piece of paper or items from quests. Items only so long as they don't need to be collected to complete said quests, though. I also store them in a certain chest in my home(s).
Now that I have the Hearthfire DLC, I collect all the useless stuff that goes in the "misc" part of your inventory to decorate my Hearthfire homes with. That includes brooms, saws, tongs, shovels, baskets, pots and pans and glasses, plates, and bowls if they look interesting enough to decorate with. I've also taken to collecting Dwemer odds and ends to decorate with, and not just the Centurion Cores, either. I also collect skulls of all types to decorate with as well. The other day for example, I discovered Wayward Pass and decided to take the Ancient Travelers Skull. Still figuring out where to put it, though.
Seems I'm not the only one who can't help but pick every alchemy ingredient they pass. I've been trying to work on that, mostly because when traveling on horseback it's a pain to have to get off to go pick some plants (am on PS3 so no mods to fix that), but it's not easy. The only thing I've managed to stop picking is snowberries, and that's only because they're everywhere and I have loads of Resist Frost potions as it is. One benefit is that I'm a pretty high level alchemist now. I have my character gather up enough alchemy ingredients to make a bunch of potions and poisons at one time and boy does that make her alchemy level up fast! Now if only she had thought to put more perks into it! lol
Speaking of that, I also can't resist picking up all the potions I see, no matter how useless or weak. The ones she can't use gets sold or given to a follower.
Same goes for things in urns in dungeons. I always pick select all, no matter how seemingly useless the items or small the gold. I also pick up any and all gold I see whether from corpses or otherwise, no matter if it's just 5 measly septims. Seems to me it adds up pretty quick so it's worth it IMHO. Same goes for enchanted weapons and armor, and mage robes. I'll disenchant them if I don't have the enchantment and sell the rest. I just can't seem to leave an enchanted item behind.
Ebony weapons and armor (mostly, though I pick up others as well to practice with) get picked up as well. Some I practice enchanting and smithing on (I'll sharpen the weapons and smith up the armor the best I can), or sell them. Sometimes to boost their worth I'll smith them up, enchant them, then sell them.
I also collect books, including books I've read, skill books, and spell books (even if I know the spell already). The books I've read and the spell books my character already knows will be sold for a few extra septims, preferably to the Orc at the College of Winterhold (I like to think she's helping to add onto the library's knowledge there, as a grateful gesture back to when it helped her), but not always.
I also practice enchanting and smithing with the extra jewelry, gems, ores, weapons and armor I pick up. If I can't use the items on my character or a follower, once done practicing, I'll sell those as well.
I have recently taken to collecting clothing of all types and storing them in my characters various homes, as well as weaker tier weapons and armor. The clothing is mostly for roleplay; my character likes to have some extra clothes to relax in her homes in. She also leaves food, ale and extra potions in her homes for that reason. Seeing as my character is always wandering from place to place, it's a way of feeling more at ease and at home for her when traveling. This is also why she's started buying homes at various points around Skyrim.
The weapons and armor are also for roleplay. She leaves them there as back up for herself and her followers in case something happens to the things they own.
Finally, no matter what Shout I find, no matter if my character can use it or not, I will always get the Shouts from the Word Walls and unlock them. Just in case I need them sometime.
I also keep various potions and potions, a few weapons of different types (right now I have Dragonbane for dragons, Meridia's sword - forget the name - for undead, and a glass sword enchanted with firey soul trap to fill soul gems with and for basic fighting. I also keep the Nightingale Bow and an Ebony bow enchanted with firey soul trap to shoot arrows at dragons with depending on type and also to sneak shoot enemies with if necessary. I keep arrows of all kinds with me as well; they're weightless after all and you never know when you need extra arrows). I also keep a set of clothes, gloves, and boots in my characters inventory in case she has to stop in at a town somewhere to stay over a few days. Again it's mostly for roleplay but it helps set the mood. Also she keeps a set of enchanted light armor and a set of mage robes on her. The robes are for if she has to regen magicka faster than the enchanted armor can which has happened before. It weighs a pound so it's not a big deal, and she'll be getting rid of it once her smithing and enchanting is good enough to make better enchanted armor.
I think that's it as far as compulsive habits go. I have quite a few too lol.
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