I would appreciate input and thoughts on what a good page layout for a mathematical article looks like in latex. What would you consider good practice? What is your own personal preference? Which page layouts annoy you as a reader? 11pt or 12pt? Standard latex fonts, or do you prefer another (commonly available) choice? What's a good compromise between traditional typographic rules (no more than 60 characters per line) and not wasting too much empty space on a page?
To make it really concrete, let's say I am specifically asking for guidance for articles to be posted on the arXiv, i.e. the article will be printed both on letter and A4 paper. And unlike journals, you don't have the luxury of choosing your page size appropriately.
(P.S.: What are good examples of layout for math content on A4-sized pages done by professionals?)