1. The things you own have a cost of ownership
Not only do you pay for the space consumed by the things you own, you pay for the time involved in mentally processing them: every time you move them around, every time you think about where they should be tidied away, every time you recall them when mentally cataloguing them, every time you remember the stories attached to them. And, if you're consciously trying to declutter, these thing are costing you time and energy every time you re-assess them and ask yourself "do I still need this?" It's exhausting. No, I don't have a neurological explanation for this (if I wanted to spin half-baked theories I'd cite other half-baked theories about how many people we're capable of having meaningful relationships with, and suggest that there's an analogous limit to the number of things we can meaningfully use/appreciate), but I know that the effort of constantly 'reading' all the symbols with which I surround myself is exhausting. I'm not saying that my mind wouldn't be full of lists and memories if I didn't own all this stuff. But at least the things that triggered the memories wouldn't need dusting.
2. You are carrying around the emotional weight of the things you don’t use.
Related to the above; there are lots of categories of things where I'm clinging to them as totems of a past image of my future/potential self: the version of me who carried on in academia and thus actually needed all those books on critical theory; the version of me who is now fluent in half a dozen languages. They needn't be major versions, but every minor revision takes up mental disk space: even the version of me who still bothers to spend two hours doing fancy nailvarnish. It is so hard to let go of those past future selves; but you can't travel in two directions at once. It's as if one of your feet is on the boat and the other is on the bank, and you can't commit to jumping on or jumping off. Even if you fall in, you're just deferring the decision; you end up treading water and have to choose whether to swim to the ship or swim to shore. The ship keeps moving out, and indecision may make you swim frantically back and forth as the alternatives get further apart from each other, or float lethargically on the surface; either way, if you don't decide while you still have the energy to execute the decision, I guess you drown.
3. You don’t learn your lessons on overspending, because you never face reality.
Less of an issue for me, as part of the reason why I have so much stuff is that I buy cheap stuff and acquire free stuff. I have to remind myself that it takes up time and space even if it's free: and time and space are money. But I certainly have wasted more money than I'd ever care to try to account for, buying more of things that I already have. Whether I've forgotten I bought them already, or lost the ones I bought already, or just want to be sure I don't run out (this sort of pre-emptive hoarding is the worst), things get stuffed into cupboards and end up becoming less than useless: I don't use them, other people can't use them, and at worst (e.g. with food) they become unusable by anybody.
4. You let yourself buy status symbols
I think that a lot of these 'status symbols' are actually a) souvenirs (the good side of the memories that things carry around with) and b) talismans invoking the ghosts of past future, as described above. It's not so much that I want to tell the world that I've read a lot (most people who are likely to see our houseful of books will already know enough about me to know that I read a lot; the rest will likely just make me feel embarrassed and awkward by asking questions like "Have you read them all?", "Are you a teacher?" and "So is reading your hobby then?") as that having books around me reminds me of things I've read.
5. You use objects as comfort
I don't think this is a bad thing in itself. (I think comfort-shopping is a lot like comfort-eating, but I daren't continue the analogy -- talking about food/diet/eating on the internet is just asking to be char-grilled alive). I buy clothes as comfort; it cheers me up to have a bright new top to wear on a grey Monday morning, or to have something distinctive to wear at a party. I buy cheap clothes from charity shops, and at the moment I have enough disposable income that I can afford to do this quite a lot. Yes, I could be saving the money instead, but the same goes for the money spent on beer, or books, or anything other than the bare essentials. But comfort-acquisition becomes a problem when you do it to excess (and that's not a hard and fast line, even for any one individual; unfortunately, sometimes the easiest way to find the location of the line is to cross it) -- when either the spending gets to the point where you no longer have the money for things you need more, or you end up with so much stuff that you can't cope. By this point, of course, the objects are in many senses no longer a comfort; but the ties of ownership are hard to break. It's harder to give something away than it is to acquire it.
With clothes, I get round this by buying cheap clothes from charity shops, wearing them a few times, and giving them back to charity shops: it's like paying to have an infinite library of clothes from which I can borrow to cheer myself up, and knowing that the library fees and fines all go to good causes. But I'm still aware that the act of shopping isn't good for me if I do it to excess, and that I could probably find better things to spend my money on than borrowing clothes.
6. You are weighed down
I suppose this one is true, but it doesn't really bother me in the sense that they seem to mean it: I don't want to live a nomadic lifestyle. I've moved house enough times to last me a lifetime, and yes, I am tired of ferrying all this stuff around with me; the most recent time we moved I 'cheated' and got the removal men to do the packing as well, because I knew that I couldn't cope with having to pack and unpack every book again. But if I get rid of things now I'd only be deluding myself if I tried to pretend that it was because then I'd be free to travel round the world with all my possessions in a backpack. In a sense, trying to live with no possessions at all would be just as much using possessions (or lack of them) to project a false self-image as buying lots of books in Japanese. Also, if I refused to acquire any possessions (without completely changing my lifestyle) I would always be relying on other people to give/lend me the things I needed, relying on other people to carry on owning stuff so that I don't have to; it's like the people who claim that they're "completely self-sufficient" when they're still using the infrastructures of society: they only have the luxury of being "self-sufficient" because other people are keeping society going to the extent that marauding hordes don't steal their yurt and kill them for food (and because they're getting paid by the Guardian to talk smugly about being self-sufficient...). One advantage of having so many possessions is that when someone near me needs something (to borrow or keep) I've often got it to hand (Owen jokes that I have one of every single thing in the world); I don't have much else to offer people, so having things to offer my friends is a service I can provide.
7. The more stuff you have the more blind you become to it.
I'd like to claim that it's fantastic having too much stuff, because every time you find things you've forgotten you had it's like a surprise present (from your past to your present, ha ha). But in practice the stuff usually just sits there, not being 'rediscovered', just being looked past: it becomes furniture, and it's not even furniture that you can sit on, or eat off.
Taking the 'surprise present' metaphor to its logical conclusion, I tried to convince my mum (who also has a lot of stuff) that we should each package up a box of the other's ornaments, bric-a-brac, etc; keep them for a year or so, gift-wrap them and give them back to each other as 'Christmas presents'. If we were delighted to receive them again then we should keep them; if they felt like unwanted gifts, then it was time get rid of them. ... She wasn't impressed by the idea.
8. If you are overspending, you will never see that money again.
Well, yeah. But the same is true of the money you spend on food and drink. And the money you give to charity. Worrying about the resale value of the stuff you buy is only pointful if you're planning to resell it. (Worrying about the lifecycle of that stuff in a wider sense is important, though. What happens when you don't want it any more: will someone else be able to use it? Will you be able to recycle it? Or will it just get thrown "away"?)
On the other hand, the idea of sunk costs can be useful when trying to get rid of stuff: if you've already bought something, you have already spent that money. If you can recoup some of the money by selling the thing, then that's a bonus; if you can't, and the thing is no longer benefiting you, keeping it will not recoup the money and in fact will just cost you more (see above). If you buy something for £10 and later give it away, you are not losing £10 when you give it away: you lost that when you bought it. If you buy something for £10 and feel obliged to pay to store it and lug it around with you for the rest of your life even when you don't want or need it, then by giving it away you are saving money.
9. Each object has a path before you bought it.
Yes, yes and yes. (This is one of the main reasons I very rarely buy anything new any more. There is enough stuff in the world.)
10. You like the idea of owning something more than the reality.
This is back to the question of storing self-image in stuff: our possessions are horcruxes where we hide the pieces of our soul. I like the idea of being the sort of person who owns a lot of books. Okay, I do also actually like owning lots of books: I like being able to pick something familiar off the shelf and turn to a page and re-read something I know well; I like being able to pick something relatively unfamiliar off the shelf and be surprised by how interesting it is (and if I find I'm not interested in it after all, I'm getting better at saying "I am never going to read this"). I like being able to lend books to friends, and I like the look and feel of a house full of books. It's warm, it's colourful, it reminds me of my parents' house, of happily losing myself in a book, of learning about myself and the world around me through books. Most of my life so far has been filtered through books. On the other hand, I probably do still own too many, and there are definitely individual books which I like the idea of owning more than I like the reality of reading them.
Part of the difficulty of getting rid of books, CDs, DVDs and other cultural consumables (they needn't be physically instantiated -- mp3s, emails, lists of bookmarks and so on all count as well, though they take up less physical space) is that letting them go is an implicit acknowledgement of mortality. It's an admission that you will not live forever, and that the inevitable corollary of this is that you will die without having read all the things you want to read. Perhaps I should try to turn this around and convince myself that by not keeping the books, I'm making the reading-list more open-ended (and thereby refusing to draw a line under my life); by keeping them, I'm merely making a papier-mache mausoleum of reading-lists around me, drawing the covers over my head.
And that's before I even get on to the deeper problem of believing we should be able to read everything, listen to everything, see everything -- the feeling that we'll die of cultural dehydration if we don't keep trying to drink from the information fire-hose (Charlie Brooker rants eloquently about this). I don't have time to write about that as well now. There are far too many books still to read.