Take games, for example. The Tower Defense game is nice enough, but no where near the quality of the Flash based versions. And this is a very, very simple game compared to, say, Fancy Pants Adventures or the like. Can processing.js. I mean, the missiles don't even point at their target, for crying out loud.
This is on the level of C64/PET character based graphics, more or less.
Again, I know nothing about processing.js at this point, so, perhaps it's possible to do much more.
Two other questions come to mind, though:
1. Flash already has its issues vis a vie cross-browser and cross-platform performance. It's not at all bad, and, because of the developer base, pretty well defined, but certainly not perfect. How does this technology compare, especially given the widely disparate support for native canvas and decent JavaScript engines?
2. What about packaging and distribution? .swfs can be self contained, making distribution to, say, portals nice and easy. A whole infrastructure for add supported apps has grown up in the Flash world, as well.