I have to admit that I didn't think about this much myself -- most of the videos are on YouTube, and I hadn't thought about how the rest of the site was run. For all I knew, it was RoR or static webpages or something.
although I have not used app engine much, it seems like you would have a lot less flexibility if you use it rather than EC2/Rackspace.
Also, Im not sure I believe statements like "its nice not to have to worry about scaling", although if it is working for KA at hundreds of requests per second, maybe it is an accurate statement
It's definitely had its ups and downs, but it managed to stay pretty stable when our traffic about doubled when CS launched.