Documentation & Reliability. I can suffer through any problem with a remote API so long as the documentation is clear, thorough and contains relevant examples.

Documentation & Reliability. I can suffer through any problem with a remote API so long as the documentation is clear, thorough and contains relevant examples. Stripe's API documentation is about the best I've seen in recent years. They get bonus points for making the 'cut and paste' examples actually cut and pastable, as they don't have instances of "stripe.com/method/<your_api_key_here>", and instead of have "stripe.com/method/my_actual_api_key", which is super convenient. Perhaps the only thing that good documentation can't trump is unreliability. This includes (obviously) an API that doesn't always return the right values, or that sometime's fails, but also includes rapidly changing API protocols. Take the time to think about your APIs early, and try very hard to keep them from changing once they're established. Nobody wants to have to refactor their code because you refactored yours.