Ask HN: What would your dream REST social API look like?

I'm in the prototyping stages of building an API for external apps (ie, mobile devices) for the Appleseed social framework, and I'd like to get some feedback, suggestions, etc. on what your dream REST API would look like. There have been plenty of criticisms of Facebook's API, what are some of the frustrations you've had? What were the best API's you've used? Do you prefer API's which are simple, but do a lot behind-the-scenes, or complicated, but very flexible? Do you prefer JSON or XML (I admit, XML will be a hard sell, but definitely try and win me over). Is consistency a must, or can the API evolve rapidly? Do you prefer username+password authentication, or is token-based authentication (ie, OAuth) acceptable? Anything I forgot to ask, let me know. I'm most interested in things from a mobile app developer perspective, because that's an arena I have little experience with. Thanks!