As a collector I have some U.S Blu-rays, specifically the Book of Eli, Casino Royale (the Daniel Craig one not David Niven) and the Upside which was annoyingly not available in the U.K on bluray so I ended up managing to source one from Canada. I would like to replace these, not because they are region locked, as I have a multi-region player, but because the spines are thinner. My sense of conformity is challenged so to have thin cases mixed in with the thicker ones is just wrong. It is probably just because I am used to the thicker cases that I prefer them. I personally find the slimmer cases more brittle and the closure on some of them can be a pain. I also like the larger spine as it makes it easier to read them on the shelves. I get the argument that thinner cases mean I could fit more media on shelves but that advantage out weighs the disadvantages to me.
U.S. Blu-ray cases are typically 11mm thick with the U.K ones being 14 or 15mm. The majority of Europe (as far as I know) sticks to the thicker sizes as does Australia. So why I hear you shout?
Digital media packaging producers within the U.K. have agreements which dictate the volume of petrochemicals used in production. In the smaller/thinner the cases the percentage volume of plasticisers, flow improvers and stabilisers changes. The thicker cases are technically greener due to the percentage (not the actual volume) in comparison to the size of the case. As far as I understand, this is not legislation but just an agreement because on occasion, I have seen U.K discs in slimmer cases.
So there it is a little nugget you can amaze (or bore) your friends with.