MySQL was never more featureful. The reason I started migrating back in the 7.1 time frame (when TOAST tables were added and you could finally store more than 8K of text in a TEXT column) was the lack of sub-selects in MySQL.
Even aside of that, Postgres was far ahead when considering basic SQL support: stored procedures, views, subselects, check constraints, triggers, actually enforcing foreign key constraints and so on.
It was significantly slower than MySQL, but it also scaled much better under load. Back then, when you had low load, MySQL would be about twice as fast as PostgreSQL but then as the load increases, MySQL's performance would drop sharply and Postgres would stay consistent.
By now, MySQL has mostly caught up feature-wise, but there's still stuff left that Postgres just does better. Also, even plain ideological reasons (community project vs. oracle open-core project) would want me to stay with postgres.
I also have anecdotal evidence that MySQL still has serious issues in the robustness department which I've yet to see with postgres.