As a founder of a startup whose Mantra is sync, I couldn't agree more. No API developer looks at API design from this angle. In fact, in a few cases, the design actively prohibited these use cases (for obvious reasons).
Another aspect is intimating error conditions. One would be amazed on the myriad different ways an API fails and many do a poor job of conveying what went wrong, and how to recover, if possible.
I'm curious, what startup that is? FWIW, I think there is a large opportunity here (make sync+collab easy => ecosystem of products that tightly sync) that can be disruptive to many SaaS businesses.
Another aspect is intimating error conditions. One would be amazed on the myriad different ways an API fails and many do a poor job of conveying what went wrong, and how to recover, if possible.