IMHO the main difference between PostgreSQL and any 'competitor' is that in most cases a software developer will quickly find not only how to use it quite properly for his use case but also why some way he adopted isn't right and triggers some non-negligible problem.

There are many reasons for this: most software developers have more than a vague idea about its underlying concepts, most error messages are clear, the documentation is superb, there are many ways to tap into the vast knowledge of a huge and growing community...