In my opinion, Postgresql + hstore + PLv8js (Javascript as a language for stored procedures, see http://code.google.com/p/plv8js/) could really rock the world if it became an accepted standard and got some people behind it.
It's not so great if you start using something and one year later see that the original authors lost interest and move on, leaving you with 10k lines of code that depend on a pray-for-no-bugs unsupported piece of software.
Totally agree. I've been building out a new system with hstore and it's been great, planning to do some benchmarking with plv8js as well. If you've got a dataset that you can conceivably scale vertically to accomodate, postgres is a solid and feature-packed database.
It's not so great if you start using something and one year later see that the original authors lost interest and move on, leaving you with 10k lines of code that depend on a pray-for-no-bugs unsupported piece of software.