It was version 7.1 and "TOAST" (native large attribute) support that made PgSQL viable for GIS and for PostGIS to come into being. That was pretty cool.
But I would mark mass-market coolness to the introduction of JSONB and the integration of full-text search, so 9.4.
https://www.crunchydata.com/blog/when-did-postgres-become-cool
@pwramsey The write-ahead-log made Postgres a viable alternative for Oracle in what I was using it for. TOAST was also great. With those two things, I really didn't have an reason to use Oracle instead of Postgres.
@alpinegreg @pwramsey IMHO the really cool thing about PostgreSQL is it's a project and not a product. This has meant lots of people contributing in different directions. Competitors collaborating together to yeild something which is better than the sum of its parts. That nature of the community has meant extensibility has always had quite high significance and enabled fast innovation.
