I'm a Flex/Flash dev and I've looked at HTML5 a couple of times recently. The main product I work on is a web app similar to Adobe Illustrator for customizing print products. You can draw vector shapes etc fine with HTML5 (or SVG) but fonts is another matter because of legal issues. Yes there are some open source fonts now but they are not of the same quality plus we'd have to go back and change all our old designs to use the new fonts.

The big issue for me is development time. ActionScript is compiled, our app has close to 50 classes and uses dependency injection. Right now I can't imagine doing it with JS.

If you're looking for a tool to convert Flash to HTML 5 http://smokescreen.us/ looks promising.