My thoughts:

• Warn if components yield/return undefined.
  Probably.
  This is probably a good idea, insofar as it would catch a lot of implicit returns, especially in generator component functions which accidentally exit. And the technique of using explicit nulls is a nice way to squash this error. I continue to hate the fact that JavaScript has two nothing types (undefined and null), but maybe we can take advantage of this feature for this case.
• Warn if generator components return early.
  Probably not.
  Do we want to log here or are there valid use cases for returning early? I’m holding out for potential use-cases where returning from a generator component is useful with keys to describe fixed async sequences that happen for a specific prop, but it’s something to think about.
• Warn if arrays/iterables have unkeyed children.
  Probably not.
  There are lots of valid use-cases for unkeyed elements in arrays, and for React, all this warning ever did was make people key elements by index. This kind of warning also perpetuated the idea that keys were exclusively for collections of elements, whereas it actually has many more use-cases like forcing a detail page to rerender on page transition.
• Warn if schedule() is called and the component doesn't yield for X seconds.
  Probably not.
  Not sure about this one, insofar as I feel like there’s probably gonna be a lot of false positives for long-running components.

I’d like to only include warnings which have a near perfect hit rate in terms of something the developer shouldn’t be doing, or those which provide a way to squelch the error like making implicit returns explicit with null. The problem is that developers treat these warnings like hard errors and will get upset if, for instance, a dependency causes errors to be logged.

One other constraint is that we’re not going to be doing build shenanigans to strip errors in production. If it’s good enough to log in development, it’s good enough to log in production, and I really do not want any divergent behaviors between the two environments.

In 0.4 we now warn when undefined is yielded or returned. Other error logging requests should probably go in new issues.