Subtle changes, bugs and feedback

Actually. not the case on the web.

Sans-serif constantly comes out top of readability tests on the web.

Perhaps when every device has 300dpi then it would be different, but for now... sans-serif wins on the web.

Serif continues to be king in print.

This. It's a combination of resolution and contrast, the main difference being reading from an emitted light source rather than reflected/ambient-lit.

However, it's more to do with what's a good font and what isn't, and Helvetica (which a fair chunk of users will end up with) is so bad IMO there's a strong case for over-riding it in design with Lucida, hell, even Verdana, rather than leaving it to browser default.