For me, the biggest problem with the whole the-browser-is-the-os thing is that all your files and data live in different locations that are sealed from each other. I should be able to transfer a photo from Flickr to Facebook etc. in some standardized way. Perhaps this can help take us a step in that direction..
Chrome has a nice feature where you can drag a file from any folder/your desktop and drop it onto a file input element on a web page.
Unfortunately, this feature does not work in Gmail (!) because they use a custom Flash-based uploader there.
This works in Safari too, so I'm assuming it's a WebKit feature.