Love, love love the Unity authoring tools. Being able to publish to the web might make for both some great games and some great webGL 'experience' type sites.
I would love to be able to play Unity games on Linux in the browser. Currently there is no Linux plugin and that kind of sucks. WebGL support would be the ultimate solution of course.
I'd love to see Unity, without the need for plugin, that would make Unity games, really accessible. But only if it's done as a first class thing, and not a fallback option.
There are other alternatives which will appeal even to 12 year olds: <a href="http://pewtersoftware.com/browserbasic/" rel="nofollow">http://pewtersoftware.com/browserbasic/</a> (based on <a href="http://kikito.github.io/luv.js/" rel="nofollow">http://kikito.github.io/luv.js/</a>)
The size of the generated asmjs files is going to be a challenge for practical uses of this (the Unity engine is pretty large), but it is a huge accomplishment.