The problem with these abstraction frameworks is the same reason they exist, get you away from the browser specific features. Are browser or platform specific features bad? If they are, then why do browser vendors invent them and why do we use them? Web sites coded for specific browsers work better than on other browsers. Try dropping an attachment on a new email message in Gmail using Chrome. Works like a native application. Cool. Try again using IE9. Oops, can't do. Microsoft at Mix11 spent a good part of the first day keynote showing how much better graphic acceleration than anyone else when you ran IE9 on Windows instead of Chrome. The new fishbowl benchmark. Google's properties act awesome with Chrome. IE integrates fantastically with Windows.
I am not against HTML5. As a developer you have a plethora of development platforms to choose from. HTML5 is just another in your tool belt. It is not the end all platform that will fix all your cross platform development woes. Every time I start a new project I make sure to repeat this line to myself. Technology does not drive my app, my app drives the technology I use. For some apps HTML5 with jQuery will be more than enough. In other cases Silverlight and Flex might work. Other's might need deep platform integration so you might use WPF. It's hard to not chase the shiny squirrel and adopt the latest technology craze but you have to think what is best for your app, your company and most importantly your potential customer or user.