All of these things (1. + 2. + HD video) have been working for me for a long time. YouTube 1080p content works fine, though I kind of doubt it's using VDPAU, so it's bound to hit the CPU (4x 1.9GHz i5, hardly cutting edge) fairly hard. I'm using the 64bit flashplayer 11 from the sevenmachines ppa.
Are you using Chrome or Chromium? Chrome comes with its own flash version.
I use a beta version of Google Chrome (latest 15.0.874.81 beta) from official repo (deb http://dl.google.com/linux/chrome/deb/ stable main). I remember it was shipped with Flash, but it worked not as good as I would like it to, so I was using 64-bit betas (not great either, to be honest), IIRC by disabling Chrome's own Flash in about:plugins. Now looking there I see only one Flash plugin, which is the one installed in my system (Location: /usr/lib/flashplugin-nonfree/libflashplayer.so). So even if Chrome's custom Flash is better, as you imply, dunno how to turn it on for a test ride now.
I didn't mean to imply that Chrome's custom Flash is better. I have no idea if it is since I run Firefox (typically) and Chromium (rarely). Like I said, I use the 64bit Flash player from the sevenmachines ppa (Ubuntu 11.04).
Are you using Chrome or Chromium? Chrome comes with its own flash version.