I have to laugh at that IE 10 thing about a "plug-in free experience."
HTML 5 has a great deal of creative possibilities. Unfortunately real world support in computer web browsers and mobile devices is still not very good. And that's despite the fact certain elements of "HTML 5" have been around for more than a decade. Google Chrome 13 scores best at html5test.com, but is still more than 100 points shy of a perfect 450 score. Internet Explorer 9 is abysmal with supporting HTML5 and Internet Explorer 8 has almost no support for it at all.
What is and what isn't a plug in? I think a lot of software developers are playing very fast and very loose with the definition. Adobe Flash is the proverbial whipping boy as of late, but it is able to do a lot of things all within a single unified plug-in. This is why Flash still dominates everything from banners ads and Zynga games to videos on YouTube.
Many of the features being touted for "HTML 5" have to be "plugged in" to the native code of the browser for it to work. Not all the browser makers are supporting everything.
Google dropped MPEG-4 support out of Chrome for the HTML5 video tag because MPEG-4 isn't open source. IE, Firefox, Safari, etc. don't support all four of the video tag formats (MPEG-4, H.264, WebM, Ogg Theora) either. So it's a crap shoot on which codec to choose for encoding your web-based video. It's forcing developers to keep falling back on using Flash Video instead.
SVG (Scalable Vector Graphics) support should have been better by now. The latest versions of Chrome, Firefox, Safari, Opera and IE support it. But IE 8 doesn't support SVG, yet IE 8 is still used by a great deal of people -particularly those using older computers running Windows XP. Adobe stopped supporting its SVG plug-in for Internet Explorer years ago. The situation with the Canvas element isn't any better.
Most browsers support all of the basics in Cascading Style Sheets. But get into CSS2 and CSS3 based tags and effects and you'll quickly run into lots of trouble without entering in all sorts of browser-specific code-fixes.
For all the performance complaints people have with Flash, the same issues are going to be there with animations based in SVG or Canvas. Get enough moving vector objects into a SVG file and it will bog down the computer system just like Flash. Put enough JavaScript events into a Canvas element or simply make the Canvas element big enough and it will lag the system. Add to this the likelihood anyone creating an animation based in SVG or Canvas will have to do a lot more programming than he would from building the same thing in Flash. Those elements in a HTML5 layout will end up in higher design bills for web site development customers.
I'm doing my best to embrace "HTML 5" at least to some conservative degree with new web design work. At some point you have to be willing to cut off users of older web browsers into to use certain new features, such as rounded corners and drop shadows on DIV boxes. Not all web design clients are going to go along with that. They want their web site to work on everything, including old web browsers. At least it's not mandatory to design sites using tables anymore.