American paperbacks are floppier than British paperbacks. For why?

Not sayin' this is a bad thing, just sayin.

i remember they used to have coloured edges on the pages too, either yellow or mauve... was that just for imports?

I gotta say Penguin UK editions have vastly better cover art than US editions

When I was a boy, British science fiction paperbacks had grainy paper, matte covers, and soft spines. My copy of Ballard's TAE is taped up with clear tape. Then there were those mysterious abbreviations at the bottom of the page every thirty pages or so.

In parallel with the rise of Cool Britannia, the British paperbacks I have bought since then have become robust, glossy, and intimidating.

Serious answer: cross-graining.

Then there were those mysterious abbreviations at the bottom of the page every thirty pages or so

Those mystified me for years as a boy, until I realised they they always had (most) of the initial letters of the title words, and somebody explained they were for the printers to know what bundle of pages went where.