Never really understood why anybody would want to play a 'serious' game in a browser, other than for the novelty factor.
There are dozens of reasons. First and foremost is the sociability. It's a lot easier to send someone a link they can click on and play than it is for them to install something. Especially in today's networked world it's hugely important to just pass links around. Also, most people are busy and they like to stay in communication or use other web services while playing a game. With the game in the browser the rest of the computer's functionality is exposed.
I don't think it's the be-all-end-all but InstantAction.com got a lot of things right. The games are hardcore 3D games in a browser and they let you use the power of web technologies to keep your friends together and communicate even outside the game to bring other people in. Have a go with Legions or RokkitBall they're fun, serious games, with great social aspects.