While I do agree that the App Engine datastore does require for some planning above what you're used to from a traditional RDBMS, I think it really just reflects a learning curve that comes with any new technology: as you use it more often, you get better at it. I think calling App Engine a pain in the ass and that you wouldn't recommend it to anyone is a bit over the top.
No, I truly meant it. Everything related to the datastore is difficult and tedious. Goodbye to incremental development, goodbye to knocking out a feature in an hour. Every single change you make has to be planned.
Adding a field to an "object"? Where do you use that object? Is there somewhere that you don't use that field which becomes too slow?
How are you going to migrate that new field? There aren't tools to help you migrate. Generally, you have to change to an Explando, add the field, load every single object from the datastore, then save every object back to the datastore.
These are two examples off the top of my head. Every change you want to make in App Engine has similar questions raised. I really did mean it when I wouldn't recommend it to anyone.
I think it's a much different learning curve than with any other technology. I'm plenty adept at learning new things, but this was just unnecessary complex, with nothing to redeem it.
I'd have to agree that Django on appengine is a big pita for the reasons you mentioned. Django is a large framework, and was simply not designed with interchangeable backends in mind. Getting it to work either requires a lot of black magic monkey-patching or foregoing the majority of built in features.
On the other hand, I have found the datastore api (and most of the other apis) to be well designed. It works very well with lighter frameworks such as tornado.
