My take is Postgres is extremely opinionated, but in such a way that it is beneficial for the long run.
PG adheres to the SQL standard far stricter than MySQL (i.e. select w/o order, ACID). From 2000-2010, it severely slowed the progress of features compared to MySQL (clustering was big topic for a while).
But the problems of hacks, speed and probably a significant one, management/owners, caught up with MySQL.
Feels like tortoise & the hare.
When I think of the term "opinionated" as it applies to software platforms, I think of things that have intended purposes in mind and optimize for the ease of those use cases, potentially to the detriment of people doing other things. It also suggests a "one way to do it" approach.
If that is the definition, I don't think Postgres fits. Postgres tries pretty hard to meet users where they are, and tackle uncommon or emerging use cases. Examples include JSON support, support for writing functions in various languages (javascript, python, perl, R, etc.), and C extensions that can hack the engine in innumerable ways.
Aside from being immediately useful just as a basic relational data store, my impression of PostgreSQL is that the "opinion" is apparent in the way that new things get done. There seems to be a culture of either implementing things well or not at all, and to avoid rushing things just because people want the new feature right now.
Could be I misinterpreted the OP's statement:
> few good people who had the right priorities and made good decisions without drama and without being overly opinionated
I attached the "opinionated" to the people as opposed to the software probably because of the way the sentence was structured.
Perhaps we're all agreeing and saying the same thing.