JPEG was originally intended to compress photos, which are pretty noisy in high frequencies. The compression artifacts don't really show if you're compressing suitable images (most of your pr0n collection, for instance :-), but if you're dealing with images with sharp high-frequency detail (especially sharp edges, I find) you'll get a bunch of crap in the high band.

PNG, on the other hand, was designed as a replacement for GIF, and had to handle all sorts of sharp-edged, computer generated web content, so it don't do that. (Neither is vector-based, so if you zoom in far enough you'll naturally get a huge mess of pixels.) PNG also has great alpha-channel support, much better than GIF's "ignore this palette entry" crap.