From a purely technical standpoint, talk about "Web as Platform" used to make me vomit in my mouth a little ... if you asked me a few years ago to pick a set of technologies least suitable as a "platform" it would probably have been those surrounding HTML. Ugh. Kludgy, poorly standardized, completely not designed for the job (HTML was effectively designed for one job and subverted for another entirely based on need), requiring lots of coding and cross-browser/platform hacks to do incredibly basic things (to the point of absurdity) and re-solve problems that were solved decades ago.
HTML5 changes this though. It's the first version of the HTML standard that shows explicit recognition and intent of the Web as an actual platform. This reflects in better APIs more properly and expressly designed for this purpose. It raises interesting possibilities.
Also important is that the browser market is more healthy and competitive than it was a few years ago, so we'll be seeing a better pace of adoption of HTML5. Having no clear "main" browser will also mitigate against tendencies for any one organization to try "embrace and extend" the standard.