I see there are some pro's like we have it somehow with Java today. Maybe we can really start an environment which suits for all browsers to run a piece of code in it, which is cross plattform compatible and makes use of all the great possibilities that we have on a linux machine. Great there!
Now the con's:
With great possibilities come great responsibilities. Where do you want to start to avoid having a super-amount of unsecure boxes? Package a ready-to-use program into the box with a "just one click in your browser to start"-button, having users who run code they never look at, from people they never saw (but for some weird reasons trust), in boxes they just one-click-download-and-run? Or did I just got something wrong?