For some reasons unknown to me a lot of people think that HTML5 will kill Flash. That's arguable for sure, but these people want Flash to die because of stupid Flash banners which slow your PC a lot and disrupt your browsing/twitting/facebooking/etc. They think that HTML5 will auto-magically make everything fast with all the bells and whistles. Oh rainbows and ponies...
The problem with Flash banners/games performance is a consequence of poor programming by banner/game authors - it is not related to Flash itself by any means. It's just that we could not create animated banners by other means. But now we can! With HTML5! I decided to show the world such HTML5 "banner" and created an entry to 10K Apart contest. It is called Plugins suck - check it out! Click Launch App button on the right and follow the instructions.
The problem is that we will see such banners on real web sites as soon as HTML5 gets more adoption. It will be somewhere around IE9 release date. And there will be one major difference between HTML5 and Flash banners - you won't be able to disable these lagging banners with 1-3 clicks like you can disable Flash ones.
This is our future of the web without Flash.