My totally unscientific and anecdotal input is that, for what I end up watching (5-10 videos/week), it's in the 50-75% range (for HTML5 videos being available).
Of course, that doesn't mean much and I always have a Windows virtual machine running for just these scenarios.
No youtube video is worth that amount of time and energy.
EDIT: That's probably why I can't convince myself to put Linux on my desktop as the primary OS. I just don't want to take the time to compile github scripts or scour the Internet for drivers/workarounds/fixes to make all of the basic computing functions I take for granted on Win7 work.
It's actually quite the opposite. Scouring the internet is what you have to do to find Windows software. With UNIX-like systems the software is more organised and more often than not packaged for you in respositories.
There is no need for Flash. There never was. With a UNIX-like system you can download any video and play it at your leisure. No slow starts, hiccups, timeouts, or whatever annoyances people tolerate with Flash.
The video is a file. You get the file and play it. Simple.
I would guess there are some video formats that Windows Media Player, not to mention other Windows video players, will still choke on. This does not happen with Mplayer (which is also available for Windows, but is truly a UNIX-style program).
And of course audio can be extracted from any video. Youtube is like a giant Napster. But the mp3's are hidden in flv/mp4/3gp/webm/etc, and Youtube has better lawyers.
My youtube downloader is 30 lines of sed and can use any tcp client (wget, curl, whatever -- there are so many). It takes me about 15 minutes to write and could probably be smaller. But there are plenty of more complex solutions, e.g., there's a nice one done in Lua called quvi.
It is trivial to set up a server that takes the youtube watch?v= url and returns the download url for the video file.
Obviously this probably makes some people uncomfortable. But this is how the web works. Anything that is uploaded can be downloaded. IMO, it's more respectable to try to be honest than lying to people that video can be "protected" from download by using some convoluted Flash scheme.
Flash is fading into obsolescence. Youtube is getting stronger every day. And the lesson from that is clear, at least to me.
And as for all thos people who love the concept of "streaming", youtube still uses progressive _download_.
If I want "streaming" I download to a file on a ramdisk and let Mplayer read from that file as the download progresses. With a fast connection, using ffmpeg to do the download will allow you to do transcoding on the fly if you need it. Mplayer gives flawless playback, every time.
The link JoshTriplett gave is a simple script, nothing that takes any time to make it usable after you download it (assuming Python is installed).
But, different strokes for different folks: and choice is usually never a bad thing. Of course, I learned long ago not to throw the baby out with the bathwater.
Personally, I have a few Windows virtual machines and an old, cheap Windows XP laptop around for those rare occasions as well as a Windows 7 dual boot install for .Net development.
However (again, anecdotally), I've not had a problem with using Linux day-to-day. Nothing has been difficult and I appreciate the power the CLI affords me.
I run a Linux VM in Windows on my desktop actually, but it's my gaming machine too. Last time I gave it a shot as a full install, I couldn't for the life of me get a brand new USB wifi adapter to work in Ubuntu. All the searching on the topic seemed to indicate that 64-bit users are screwed (which was me), and 32-bit users needed to use a driver emulator and dig through generic broadcom drivers to test.
