The HTML 5 technology stack is not well positioned to do everything Flash can do though - and I pointed out three key places where that's absolutely the case.
The biggest point though is that HTML creeps slowly forward through a standards process that is laborious at best. Flash, being a de-facto standard controlled by one company doesn't have to wait - it can push and prod forward and let developers explore new ideas and create new experiences. Even if you don't care for Flash, you should appreciate what it does for pushing the web forward. Sure there are missteps - I think Flash ads are awful too, but the work and because of that HTML5 and canvas will be used to build just as annoying ads in the future.