Based off their browser - Chrome
http://www.pcworld.com/article/168028/google_announces_chrome_os.html?tk=rel_news (http://www.pcworld.com/article/168028/google_announces_chrome_os.html?tk=rel_news)
Microsoft should be nervous about this one. Also Google Wave - integrates tweeting with email across platforms.
At the very least it will engender a little bit more competition in the space.
I think microsoft has to learn 2 things: Web based applications, and developing good solid native applications for both their other platforms. Like they did with their Office suit on the Mac.
And a third thing is every decade they need to develop a new OS from the ground up and not worry about reverse compatibility.