It's not intrinsically tied to the ORM. Sorry if I gave that impression. However it has many helpful shims and subclasses that interface with the ORM.

I'm looking at ModelResource as my example.

If you aren't working with a NoSQL database or a search engine, or grabbing data from an tertiary resource then the sheer complexity found in TastyPie might not be for you.

Django-Piston doesn't offer many features, but its dead simple in its usage.

Yep, I agree. Tastypie has a steeper learning curve, due to beeing more flexible. When I used Piston, jespern was not actively maintaining it, but using a private branch for bitbucket.