Looking at the django-rest-framework API examples, that's one thing I don't see, but is (at least for me) fairly critical.
One thing that does seem to be a bit lacking is the documentation. I've used all three django frameworks though (piston, tastypie and django-rest-framework) and django-rest-framework wins in my book.
This is a bullet from the django-rest-framework page I linked above: "Modular architecture - MixIn classes can be used without requiring the Resource or ModelResource classes."
Here's one of the examples from django-rest-framework that is not tied to models: http://django-rest-framework.readthedocs.org/en/latest/examp...
And also, from the Tastypie docs: http://django-tastypie.readthedocs.org/en/latest/resources.h...
I want to expose arbitrary view methods to GET calls, not linked to any model, and not have to write a new class for every view I want to expose. Basically I want views to return the correct filetype (json, html, xml, etc) when requested. This is basically making django a little more rails-like. I'm leaning towards django-dynamicresponse currently.
I really like how the three you mentioned tie into Django's forms though, and that's quite a feature to give up.
You just define a view that inherits from djangorestframework.views.View, and implement the action methods (get/post/put/delete) that you want. These methods must return a dictionary, and the framework handles the part of dispatch and render the view in the proper format.