Adam Bosworth contrasts his definition of the platform to the one of Microsoft (emphasis is mine):
The platform of this decade isn't going to be around controlling hardware resources and rich UI. Nor do I think you're going to be able to charge for the platform per se. Instead, it is going to be around access to community, collaboration, and content. And it is going to be mass market in the way that the web is mass market, in the way that the iPod is mass market, in the way that a TV is mass market. Which means I think that it is going to be around services, not around boxes. I postulate, still, that 95% of the UI required for this world will be delivered over the browser for the same reason that we all still use a steering wheel in a car or have stayed with << < | > >> for so long. Everybody gets it. But this will, by definition, be an open platform because the main value it has is in delivering information and communication. Notice that the big players, Amazon, eBay, and Google have already opened up their information through Web API's. It is Open Data coupled with Open Communication built on top of Open Source that will drive the future, not Longhorn.
I believe in most of it -- especially in services tuned for community, collaboration and content -- except for the "95% of the UI required for this world will be delivered over the browser" because, unless the web standards grow up to offer a significant improvement in terms of UI, the user experience coming out of a browser is still a disgrace compared to what any modern OS can offer to desktop applications. See iTunes for an example of what can be done with a desktop application connected to web services.
P.S. the title is purposely connected to my earlier posts about Browsers War II, since it's pretty clear that Microsoft has seen the "web as a platform" as a significant threat against its "Windows as The Platform" model, and therefore had to move from ignoring it to "embrace and extend" it.