Aral Balkan addresses the most pressing problems with Google App Engine today:
You build an awesome new app on Google App Engine. You tell your friends. They tell 1,000 of their friends on Twitter who tell 1,000 of their friends and then, suddenly, you have all these developers hitting Google App Engine for the first time to see your app. Paradoxically, by doing that, they trigger the “intelligent throttling” “feature” in Google App Engine which freaks out and shuts down your app with an “Over Quota” error — effectively making the “Over Quota” message the first impression most of your audience has of Google App Engine. Not good.