I found colour block quite useful when I was looking for a concert "spring colour" top the other week. But they had the size cubes on, too.
One of the most successful charity shops I have known (Comrie Cancer Club shop) is your proverbial musty place. But oh, wasn't it busy all the time? Books 50p or 3 for £1, if you don't like it, chuck it back. Got some great bargains (all 7 James Herriot books in one band) and new to me authors that I wouldn't have tried if the books cost £2. We only tend to buy a book in places like BHF/Red Cross if we really, really want it. At the aforementioned shop we also bought about 20 small cut wineglasses, 20p each, a steal. We just rounded them all up, but many years later donated at least half of them to another charity shop, still making up a good set.
Space by the bookshelves to peruse and not be in other people's way.
Gift aid annoyance, too. Not only having to register but also having to give an estimate of the value, eh?
A way to drop of a few bits without having to wait for the cashier to go through an endless ritual with a few well-meaning customers and their purchases.
Too high prices for some semi-branded stuff. Like an M&S shirt for £3, which was well shrunk (from 22 to comfortably fit my size 18). I know the shirt would have been £8 max bought new, I think that's still a high price for second hand. But I needed it (see "spring colours" above).
I hate the jumbled look of clothing, sorry, tops together, trousers together coats together. I don't ever go to shops like TK Maxx due to a) the jumble and b) something never being in your size (or buying thinking it will work while you know better, really).
Stuff so way out of fashion nobody's going to buy it (long pointy toe murderers). There may be a customer for it, but it's more likely going to be scrapped. The amount of stuff that is ragged before it even hits the shop, and afterwards, is huge. Even then there are some who would argue these items could be sold for 20p each. I don't know the truth of that.