Here's more than you probably wanted to know about font rendering and why Windows, Mac, Linux, and Adobe Reader are different.
Texts Rasterization Exposures
I found this via Joel Spolsky in connection with his earlier post:
Font smoothing, anti-aliasing, and sub-pixel rendering
I'm afraid most of the time I don't even notice these fine details. But I have struggled with text sizes and layout issues in Windows. And really bad rendering can be quite annoying - Open Office (on Windows) used to be pretty bad.