The idea of running all content without needing any plugin (and neither using things like native client) is nice, but its at best a distant theory for a lot of more in depth interactive media mashup content.
I see a lot of actually wrong things posted in this thread, so let´s go through some things:
-Just because html 5 has a fancy name and logo its no set in stone standard which is final and usable and functional in all browsers today.
Rather the opposite, have a look at just how to implement more in depth video playback and streaming controls or music control in a few browsers in html 5 manner and you`ll see how different that is done in a few different browsers and how much that is not a sense making or final in all cases well working usable standard.
Besides that, a few companies can join in and define something as a standard but that only makes it a standard by definition, not a standard that is adopted and used.
Flash, no matter if one likes it or not or whether it is a proprietary plugin tech or not is the web standard for presenting in depth 2D interactive media mashup content on the web. There`s no discussion about that based on facts because there is no better alternative or even just one which allows to do all that can be done in flash right now.
Who says flash is now dead or html 5 is ready to replace it on all fronts has no idea or is intentionally lying.
Just as example: Most money in the game industry is made with micro transaction based games on the web. In which technology?
Most times flash.
Are there game creation tools for html 5 as good as flash or unity ide to make the same kind of games in similar amounts of time and have them run at same or better performance across all browsers?
No.
Most money on other web sites is made with advertising. Advertising is largely presented in which technology on the web?
Flash.
So yeah…
Next up, me myself and many others using things like unity and unreal expect way better performance than flash offers as unity and unreal make proper use of the gpu (unlike flash (other than for video), which will only offer that fully for stuff making use of the new apis for the next flash player)
Still, HTML 5 stuff is on many ends not better performance wise than flash when doing similar things, the contrary, its actually even worse for many things.
Also all those who complain about flash performance should be happy about flash being so dominant, thanks to that it is used for such a high percentage of advertisements. Which means you can disable the flash plugin or install a click to flash like plugin and can browse with a lot less blinky annoying banners. If some time in a few years html 5 is ready to be used and gets used as much for creating the same kind of annoying blinky banners, well, you don’t have much of an option to disable those while still making the website work well (especially not on browsers which don`t offer plugins like click to flash, adblock etc 
)
Regarding the Windows 8 remarks:
Windows 8 comes with two versions of many apps, regarding this topic it comes with two versions of IE, one is the version that comes up when launched from the classic windows interface, that one has full support for plugins, so also unity, flash etc.
The other one is the stripped down one that comes up when one launches IE from the metro start menu, that one doesn´t support plugins but has menu item in the corner to open the page in the full IE.
So yeah…
Me myself i complain often enough about lousy flash performance, and Adobe is the one to blame for Flash´s worse getting reputation by not offering upgrades which improve general performance for all content years earlier,
but it just doesn`t make sense to jump onto the “let´s kill flash, html 5 is so much better” bandwagon while that is just no realistic alternative for a lot of interactive media mashup content and those companies yelling it loudest often have other reasons behind the move than just wanting a better performing plugin less alternative (which doesn’t exist in a form that runs as well across all browsers).
To me it looks like flash usage will be less and less for basic website content and yes, more and more basic things will be done in html 5 where applicable; but for a lot of more in depth stuff, people will still do it using a plugin technology or deploy an app for mobiles depending on what´s more sense making from case to case, and that´ll go on like that for quite a while.
Maybe let´s check in again on this topic in 2015 when we can see how much html 5 replaced all the plugin technologies properly in the sense of being able to do all the things one could do in the plugin technologies =)
