A few months ago, I published an article here explaining why the DSi/iPhone/iTouch/iPad would never get Flash. Well, my take on it was said by Steve Jobs on "His Thoughts about Flash".
"The move to multitouch is not supported by a mouse pointer-centric Flash sites that use rollovers and other desktop behaviors and since these will need to be re-written anyway, Jobs believes they might as well be re-written in open HTML5, CSS, and JavaScript formats." - Steve Jobs
This is from STEVE JOBS. The same issue goes with the DSi, even though it's not multitouch it's touchscreen based and does not utilize the use of a cursor.
It's true, HTML5 can't do as much as flash, and thats what you would expect.
HTML5 does have loads of useful features though, the <video> tag can be intergrated with JavaScript to produce stunning effects that flash would only have done much more slowly.
The <video> tag can be used without plugins. That means no costly developer packages, no updates and essentially could be played by the DSi.
Coming back to what Hull said, I thought HTML5 YouTube does use flash for the interface, but mabye I'm wrong on that.
Flash is also very buggy. How many times have you seen your Wii, PC, or Mac, crash because of Flash?
Crystal - Don't worry about YouTube not working without Flash. The developers have set up all the videos to play with HTML5 video, which is used by the iPhone, iPad, and all modern browsers except for Internet Explorer. As for IE, rumors are floating around that version 9 will have support.
For those on a computer, try the following link and tell me HTML5 can't do what Flash does: