It's also worth mentioning that unlike AppEngine the Heroku stack is a unix based environment (call "heroku run bash" and boom, you have shell access) and its components are all open sourced, so it should be relatively easy to move from there to a VPS if you ever need to.
Other than that, Heroku comes from multiple years of supporting Ruby (and some Node.js), their main page still states "Agile deployment for Ruby and Node.js", there's a lot of Rails/Sinatra/Rack specific stuff in their docs, and the Heroku command line tool is a Ruby Gem. In other words, this is not an "Heroku for Django", like Djangy was, it's become more like an "Heroku for anything, with a strong culture of Ruby", and IMO that's extremely exciting: we're talking about access to multiple languages and tools on the same easy-to-use cloud.