It's not just the database itself (and that's awesome on its own right), but it's also all the peripheral stuff: The documentation is seriously amazing and very complete, the tools that come with the database are really good too (like psql which I still prefer to the various UIs out there).
Code-wise, I would recommend anybody to have a look at their git repo and the way how they write commit-messages: They are a pleasure to read and really explain what's going on. If everybody wrote commit messages like this, we'd be in a much better place what code-archeology is concerned.
Patches from the community are always patiently reviewed and, contrary to many other projects, even new contributors are not really required to have a thick skin nor flame retardant suits. The only thing required is a lot of patience as the level of quality required for a patch to go in is very, very high.
Finally, there's #postgresql on Freenode where core developers spend their time patiently helping people in need of support. Some questions could be solved by spending 30 seconds in the (as I said: excellent) manual and some of them point to really obscure issues, but no matter what time it is: Somebody in #postgresql is there to help you.
I think there's no other free software project out there that just gets everything right: Very friendly community, awesome documentation, awesome tools, and of course and awesome product offering to begin with.
Huge thanks to everybody involved.
Also: Huge YAY for jsonb - I have many, many things in mind I can use that for and I have been looking forward to this for a year now.