It seems to me if you are already a web / website designer then you will welcome the changes in HTML5 and probably find many of them useful in expanding the functionality of your design. However if you come from a Desktop programming background then you will probably be underwhelmed.

As far as it being a Flash killer, give me a break. Adobe Flex & MS Silverlight which are the 2 main rich web technologies out there enable you to do far more than HTML5.

HTML5 even in conjunction with Ajax wont give you as much functionality as Silverlight or Flex !