I love that the video starts buffering but doesn't start playing when you load the page.

I don't know that I've ever seen that before. You either get buffering and playing when you load the page or you need to click play before the video starts buffering.

Even more interesting: it does not buffer the video all the way, on my machine it reliably buffers 10 seconds of video then stops until I hit "play". At which point it loads all the rest of the video (so it's not that it only has a 10s buffer, they explicitly selected to preload only that amount of data)

Isn't that an undesirable behavior? You are consuming network bandwidth without even using (or knowing about) it?

What if you are on an expensive mobile data plan?

Then don't activate flash/video/etc unless you activate it. Dolphin HD browser supports this behavior.

And it's absolutely desirable behavior in any site that gives you related videos (such as YouTube) as you get to open multiple videos and have them buffer in the background without disrupting the flow of you consuming content.