The Serif looks questionable to me as well. The vertical line on the lowercase 'h' in particular looks like it has artifacts, both too thick and oddly blurred. The Greek serif has a lot of the same artifacts as well. The sans-serif looks fine, but something looks pretty wrong with the serif. I also thought it might be a client-side rendering issue (I'm using a Mac), but the demo page has the text prerendered into an image.

edit: Found a different demo page that renders the webfont client-side instead of showing images, and looks much better to me: http://www.google.com/fonts/specimen/Noto+Serif. Maybe it's just that the pre-rendered specimens are made with a poor rendering engine?

It looks awfully blurry in Firefox on Linux.