The attack surface is something I didn't consider, and presents a tough challenge. If the framework were to implement the transport layer in a predictable way, I think it still might be a net win. It could build in all sorts of automatic good protection against XSS and CSRF and have even better control than today's frameworks since it can validate on both sides and know what to expect. The ever changing attack surface is a problem without a doubt, but the usual vectors can be better protected against, and the visibility and control issues can be mitigated with inline directives.
With regards to SEO, I disagree. There are two issues at heart here. Firstly, some web applications are single user apps with no publicly indexable data (ex Mockingbird, Basecamp), and optimize landing pages to direct users to use the application and explain why it is worthwhile. That's the data they want in Google, not the data from within the application. For these types of apps the issue SEO isn't that big. The issue is big with apps like Hunch, where as you say there are many states and non page-like semantics. Take a moment to examine the data that these kinds of apps want indexed and searchable. It's usually central to the app, the meat of the whole thing, and in the case of Hunch, available as a discrete page because it makes sense. This leads me to believe that you can without too much difficult come up with a URL scheme for representing it that either does or doesn't have an anchor in it. Thats the central idea, is that the routing table is the same on both sides of the wire. The first page they visit can be rendered server side and all ensuing pages can be rendered client side using fragments after the "#", and the Googlebot can index the pages as they are all renderable server side. This also ties into your third point, that the routing paradigm used by Google and everyone now is the only way to go. I really don't know how to solve the multiple state vs url segments problem while remaining indexable, but I believe it can be done. Do you disagree that the paradigm is no longer as useful?