Agree with everything mentioned about the 'benefits' of not having Flash (no flash ads, no battery drain, overall better web browsing experience). Also, if it?s a major concern for you, there are a bunch of seriously (seriously!) awesome YouTube apps available for WP7, which are far better than just having the mobile site (oh, and we do now have the YouTube HTML5 site too).
And I do hope the world does move away from Flash to HTML5 eventually (it?s starting to..), but in the mean time it is a bit annoying for me personally to not have Flash on my phone:
I avidly follow music blogs and almost every site I follow uses these little embedded flash players for streaming audio as well as videos from a number of non-YouTube sources. So for me it does bug me, but definitely not enough for me to ever even remotely consider moving to android?
Also, there are plenty of third party apps that let you enjoy other 'flashy' sites like VEVO, Vimeo, SoundClound, and probably many more, with others sure to keep coming? Plus, I may or may not be working towards some apps to bring such sites to our platform in a cool and ultra metro way (in case that's a deal-breaker for you).