Don't get me wrong, HTML5 brings some good technologies to the web application developer. Nevertheless, I think there is a lot of hype as HTML5 doesn't really bring anything new to the table.
First of all, there are multiple type of applications that we are all well-aware of;
If you are building a consumer oriented application which doesn't use any of the native functionality of OS, then I would recommend the application to be built in HTML5 and all the RIA fanfare that comes with it. You can best view this in the mobile space where developers build either native or web apps depending on the application requirement. Remember that web application (HTML5 ) do not have direct access to printers, USB port or any other hardware such as Bluetooth and network services. Simple word processing application that can be developed in HTML5 such as Google Doc (where this blog is typed from). I haven't come across any serious application written in HTML5 or the likes yet (JavaFX, Flash and Silverlight are not HTML5).
Business ApplicationCritical business application such as POS which requires access to barcode scanner as an example can't be written using HTML5. You can have a native application delivered through the browser such as Java Applets ( or JavaFX) using webstart. Java applets can access OS features and hardware and provides another layer of security. Business application are delivered in controlled environments, for example, the application can be deployed on Linux desktop in company "a" environment only. For as much "fanfare" one might create around HTML5, these type of application will not cease to exist.
ConclusionHTML5 is a promising step in the right direction to building scalable robust web application but it will not obliterate desktop applications, not today, not tomorrow, not ever (really!?). We can already see it in the mobile space where developers rather write native application so that they can utilize OS features and hardware. Web applications cannot access you local files directory (I am not talking uploading a file to a site) to read or write. A simple operation such as reading available space in a directory or writing a log to a local directory is not possible (again I am not talking about downloading or saving a page). Therefore, developers should not believe the hype. HTML5 is not the silver bullet and it is a shame when companies such as Adobe sends mix-messages by discontinuing their Flex offering. Anyway, Adobe has never been a big player in the enterprise desktop application market.
If you disagree with my points, feel free to share your thoughts.