I find App Engine and Heroku to be in stark contrast in this respect. App Engine requires you to code your app in a particular way to work on their platform, a

I find App Engine and Heroku to be in stark contrast in this respect. App Engine requires you to code your app in a particular way to work on their platform, and that way will (to a large extent) only work on App Engine. Heroku, on the other hand, requires you to code your app in what is pretty much the recommended way to do Rails apps anyway. While they do impose a few limitations that aren't present on other platforms (e.g. no local writable storage), accommodating those limitations wouldn't prevent you from moving to another platform. There's no proprietary Heroku Database API that you'd have to reimplement in order to move to a physical-server-colo setup, for example.