I think one of the main advantages to GAE is the time to get a site up and running. There's zero server setup, installation, or management. This definitely helps you get out the door fast. You're right though in terms of data mining etc... I work for a keyword intelligence site that definitely couldn't run on app engine. We run on a Microsoft stack like stackoverlow and are extremely lean for what we do. However, if I were to start a web 2.0 type site where most of the value is created by users i.e. facebook, twitter, wikipidia, craigslist, or even stackoverflow, app engine would be the way to go.