12pt Times New Roman is the standard for publishing, so that's become the go-to size for a lot of websites. It was plenty big on 800x600 displays, but I agree that it's generally too small for today's resolutions.

However, the solution isn't to pick a new, slightly larger point size and just set everything to that, it's to use media queries to set an appropriate size for whatever medium the content is being displayed in, and to use sizes relative to the user's default rather than specifying exact point sizes.

You can also use absolute sizes (in and cm in CSS) so that the browser will display the font at the given size without regard to the display resolution or DPI.

Edit: Also, I highly recommend Readability, but obviously it'd be preferable if people just did the right thing to begin with.