Great points. The design issue is a big one, and I think the main reason is that PSDs can be translated to HTML and CSS easily, but extracting a theme for a set of widgets is much harder. The widgets give you rich interaction and an easy way to set up complex layouts, but as you said can be very rigid and not allow easy modification of their behaviour. And I agree, I end up longing for the simple world of HTML and CSS where I know exactly how to change something instead of fighting the framework. I think thats something that will be remedied if a framework like Planet or my hypothetical one reaches ubiquity however. If developers learn the ways the framework works, just like how they learned the ways HTML, CSS, or Rails works, they'll be ok with extending the widgets to do what they want.

Another good point about routing, and this I'm admittedly fuzzy on. Above mechanical_fish mentioned some serious SEO issues that arise from moving away from the standard paradigms, so I'm not quite sure how it should be solved.

All in all, Planet looks neat, thanks for sharing!