While this solution obviously won't work for most users, personally I just reach for youtube-dl (http://rg3.github.com/youtube-dl/) when I run into a video that YouTube won't serve with the HTML5 player. No flash, no ads, and no buffering.
Safari's ClickToFlash/ClickToPlugin is absolutely awesome as it replaces the basic placeholder with a (apparently QuickTime X) player with an overlay offering source format selection (including Flash object, and a download option). Chrome has a similar plugin, although less advanced (and generally more flaky). Firefox's FlashBlock is so limited and looks like a stab in the eye in comparison.
