Documentation plain and simple. If someone can't be bothered to document it, then I can't be bothered to use it. If two libraries have good documentation then you read up on both and decide which fits your mental model better.

I tried RoR before it had hit version 1 and the only documentation was the prag Prog book. Looked through the Zope and Django documentation and found the documentation to be better quality. Django just fit my mental model better so that is what I went with. I don't regret the choice, sure I have found some corner cases where I and it haven't agreed but that happens any code that you don't write from top to bottom.