The big point is about separating the innovation over organizing the user's experience of the web in general (the Browser) vs. innovation in the display of web sites (the Renderer).
The Chrome Frame approach allows browser makers to preserve their revenue models and their UI interaction models, at least in the ideal -- there are some specific issues to work out with the Chrome Frame model, like password/form data storage. But I think the direction is a good one.
It also solves the issue where a web application was developed a few years ago and is not going to be maintained any more. If the browser allows multiple rendering engines it makes it possible for those old web apps to continue to work.
Old enterprise/business applications in particular can continue to work, but we still get to move the new web experience forward.
From a browser market share perspective, this clean split of responsibility could allow a newer browser like Google Chrome to get more market share. It could use the reverse idea: embed the IE render engine in Google Chrome. Then, take that to all the IT administrators and say, here is a newer, safer, more secure browser for your company that will work on older machines, is free, and can still allow your old business web apps to work.
That may not work, since most people are attached to their browser chrome vs. the actual renderer. In those cases, the plain Chrome Frame approach of installing a plugin for another renderer works to allow better, faster web apps experiences.
One concern: user choice or control. I think it actually leads to more user control: the user gets to keep the browser chrome, their organizing model for the whole web, intact, but gets to use more of the better parts of the web. And for areas where the user does not have control now (in business environments that need old apps to work), it gives a way out to use more of the web.
Of course, there should user controls so they can set preferences and overrides for the renderers used. At a minimum, business IT groups will need it to configure the browsers to use old renderers for older in-house business web apps.
So, some tweaks to the basic mechanics I would like to see (realizing that I have no concept how hard this work would be):
1) Make sure the Renderer works well with the Browser: for instance make sure that saved passwords/form data works well no matter what Renderer is used. Make sure the split is clean.
2) Change the UA-Compatible thing to be more Renderer-based feature sets vs. browser version numbers. So, something where the developer mentions the capabilities that are desired:
<meta equiv="X-UA-Compatible" content="addEventListener=1,svg=1">
If the developer has a suggested browser renderer, it could place that at the end, sort of how CSS font names can start with generic names, then get more specific:
<meta equiv="X-UA-Compatible" content="addEventListener=1,svg=1;gecko=1.9.2">
It would be good to *not* use browser names in the tag, but rather the renderer engine names/versions. Ideally though, just list capabilities.
The above syntax is not exactly right, but just to demonstrate the idea: focus on telling the browser the capabilities the page wants, and use render engine names, not browser names.
3) Make sure the user can override the choices made by the browser. The pref control does not have to be obvious, but should be there, so the user has the final say.
In summary, as a conversation starter, I like that Chrome Frame has really tried to highlight the difference between upgrades in renderers vs. browser interface.