A bit literal-minded of me, I know, but I'm that kind of nerd.
Everywhere in Europe, Africa, Asia (except the Philippines), and Australia, the standard size for everyday-use cut-sheet paper is A4 (210mm x 297mm – approx 8.3" x 11.7") as defined by ISO 216, as opposed to Letter's 8.5" x 11", and it's pretty inconvenient for an ordinary person to conveniently obtain Letter size paper (i.e. by just walking into a shop and buying a ream of it).
A4 has 3% more area than Letter, due to being 6.5% longer but only 2.8% narrower; is anyone going to complain if I use A4 as my paper size instead of Letter?