Users don't read, they skim. If they can't skim, they leave. We can make our websites skim-friendly by establishing rules of good typography.
Body copy should be 16px.
This might look huge in your Word Document, but it won't on the web. You're not writing a thesis paper for a professor, you're writing to sell.
Line spacing should be 1.5.
I know what you're thinking. "But the page will be so long!" That's true, but pixels are free and users have known how to scroll since the nineties.
Line lengths should be 52-78 characters.
Remember newspapers? (I'm sure someone still reads them.) Ever wonder why they use multiple columns of text instead of just one really wide column? It's because there's an ideal width to improve readability. Any given line of text should be between two and three alphabets long.
Use almost black text on an almost white background.
At a glance, the user will think it's just black on white text, but the text will look slightly smoother than usual. This is because we've given the operating system's anti-aliasing some breathing room to do sub-pixel hinting on our font. (The explanation is a bit technical for me, too, but I know it works.)
What are your biggest annoyances when trying to read a website? Tell us on Twitter: @ethercycle