Based on my experience with other developers, they can't figure out how to login to the database and create users, databases, etc, initially, IMO mainly due to the default config doing ident only by default, and not allowing passwords to be used unless you setup pg_hba.conf first. I think that gets into sysadmin territory for the masses of developers, that just want to install, run a few commands, and go.
And, even once you've got password logins working, authentication works really strangely in postgres. Every database has a single "owner" (based on who created it) who has full permissions. If you want to make more permissions available to another user, you have to grant them manually, on a table-by-table basis. Want to grant SELECT on every table in a large database? Sucks to be you!
Compare this to MySQL, where there's just a single magic table of permissions, and you'll begin to see why a lot of system administrators prefer MySQL to Postgres as well. :(