Since we are on this topic, I have a public service announcement: even though light Open Sans looks great and is trendy for headings, using a light font weight for body text is a bad idea because it makes it difficult to read. I keep coming across examples on the web, always with bad results.
Open Sans itself is a nice font, I have nothing against it. As a web font it might get rendered improperly, but that is an issue with the browser and the platform.
What I was talking about -- unlike, say, Arial, Open Sans also has a light weight (font-weight: 300) variant. That is now popular with large headings BUT I do not find it at all readable as body text (in Open Sans or most other fonts). Wordpress.com is a good example, as they do NOT make this mistake.
Thank you.