I have had the OP's exact issue. I first noticed it with Flash, so I installed the Safari extension, "Click To Flash", assuming it was only a software issue at the time. I then proceeded to watch a YouTube video with with "Click To Flash's" HTML5 conversion script. Yet again, streaming through Quicktime's HTML5 player, the entire system crashed. Even further so (and unlike the OP), I downloaded a video and played it via Quicktime X and, once again, my system crashed as the OP described it. This is most likely not a compatibility issue, the beta OS just has a few bugs to work out with the NVIDIA GeForce 9400, and upon myself and many others filling a bug report with Apple about this, everything should be worked out by the GM. Although disabling hardware acceleration does seem to help with the Flash based crashes, Mountain Lion currently has a problem with playing back video (via the NVIDIA GeForce 9400) throughout the system.
I only noticed this on both DP2/3. I have an iMac Early 2009 24" Model 9,1.
I should also note that this happens at a complete random, and is with all video sources tested (not specific to YouTube).
- Andrew