First, we do not really know the first payment/milestone, maybe it's just $1M.
Meteor itself is an amazing technology, very well marketed by obviously smart guys—their Marketing pitch few weeks ago was just awesome and far beyond any other new JS framework. And I understand that Meteor gets very positive feedback here on HN due to their great communication skills and YC affiliation
But it has severe drawbacks:
=> While employing Node as core they surprisingly ignore the well established npm package manager which is one of the best package managers around. This is bad and there's no excuse because it leads to fragmentation of the still young JS server-side landscape dominated by a lean and modular-driven Node which is just the smartest way to establish a real ecosystem—the one-size-fits-all approach is aged and that's Meteor. I assume they did their own package manager due to their upcoming business model (which will be introduced very far in the future if their ecosystem is once established), maybe they'll take license fees or demand support fees or whatever of everyone who wants to actively participate as contributor in the ecosystem. They couldn't do this with the npm. And by choosing this path the can lock out competing frameworks: if Meteor would just be a package in the npm ecosystem the opportunity costs of changing to other realtime frameworks in the npm world wouldn't be that high because changing the framework wouldn't mean changing the entire ecosystem.
=> As long client-side JS is delivered unprotected to the browser you will never have the one-code-base-or-name-space-covering-front-and-backend approach. This approach doesn't provide any security—client code could do any shit to the server side—and others who tried made great products too but couldn't get any traction (nowjs i.e.). You will need always to separate both. They promised to come up with solutions like authentification or signed data, but then we have again more communication overhead than we would have if just separated those layers. This drawback isn't as huge as the first one, it's a technical challenge and thus, I appreciate any efforts to solve this problem.
Meteor was at the beginning a great tech demo, now they want to get serious and I doubt (and hope) that they won't succeed. Mentioned drawbacks are the main reasons I won't use, support and even advise against Meteor (as much as I like these guys and YC but sorry). They do not seem to contribute in any way to a great and existing ecosystem called Node but using it as their core to build a new competing one with monetization reasons in mind and a severely flawed architecture. Now, they obviously need and will use the money for PR and paying/incentivizing devs building the ecosystem and this competition between ecosystems (pure Node/npm vs Meteor) which is basically about winning the best devs will lead to further fragmentation and at the end no large ecosystem could be established and server-side JS failed. No, thanks.