When I choose a new platform, I try to assess how long the platform my survive. APIs, ease of development and ease of use are important too. But when I am going to spend many hours trying to learn a new platform, I try to as sure as possible that the platform is going to last for at least 5 years.

Now it is easy to judge this for well established platforms like Linux, GitHub and software in the similar league but it is not so easy to judge this for new upcoming platforms offered by emerging technologies and startups. For the latter I look at what people are talking about the platform online, how much financial or community support it has got. For example, I would be more willing to put effort into learning a new open source platform with 1000+ GitHub stars and 100+ forks than I would be willing to learn a platform with 20+ GitHub stars. Similarly, I would be more willing to put effort into a new startup's platform that has got several millions of dollars of funding and good reputation than I would be willing to put into a platform with little to no funding. These are not perfect metrics but they make a good start.