What makes a platform compelling?

I posted a few weeks ago asking how important API usability is. At least three people believe that it's important. Thanks to Bing, I found that others seem to agree too: David Koelle on API usability, API usability people and resources, Mathieu Jacques' article on CodeProject, Carnegie Mellon University API Usability project, and there is also a great discussion on API design vs API usability.

But the thing that I really want to get a handle on is how important API usability is compared to every thing else that you need to factor in when choosing which platform to target. Think about reach for example. Would you take a risk on a new platform that has very little adoption, but which has the best APIs you can imagine, in the hope that it will attract developers, which in turn will attract customers once those developers start publishing their apps? Or would you wait until adoption of the platform increased, no matter how good the APIs are? Or would you decide to target a different platform, one that has greater market share, but which doesn't have as good a set of APIs?

For the sake of discussion, let's assume that 'good' refers to the usabilty of the APIs, not the capabilities. Assume that the capabilities are identical across different platforms.

How do you decide which platform to target? What other factors are more important than API usability? Less important?