Am I the only one surprised by the fact that KA is hosted on Google App Engine?
Our backend does much more than just serve static files -- you'd think that we don't have to do much but we keep track of video and exercise progress, summarize progress by user and by coach, provide video recommendations, host a Q&A system with notification support, award badges in real-time while avoiding costly datastore queries, handle mailing lists and email subscriptions, and have support for having a public profile to show off your progress, among other things.
We handle hundreds of requests per second and it's really nice to not have to think about scaling very much -- with any other solution, we'd be doing a lot more system administration.
It's definitely had its ups and downs, but it managed to stay pretty stable when our traffic about doubled when CS launched.
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