Mysql v.s. postgres (my take)
* It's GIS plugin is the most complete I've seen of even commercial tools.
* The advanced index support [now] bests myql (at least as of 9.2, which finally supported covering indexes). I'm sure Oracle has a couple advantages for specialized needs, but I'm not familiar enough with Oracle.
* The system defaults to highly-robust, but can be tuned down (based on hardware / risk-tolerance) to higher performance
* Using RDBMS raid solutions, you can scale moderately
* Using classic sharding techniques, you can scale along with NoSQL solutions (of course, this isn't postgres specific)
* VERY good graphic editor (pgadmin), gives VERY good remote diagnostic support.
* Pretty good integration with 3rd party credential systems
* Great support for schema's (e.g. namespaces) "select * from project.mytable", unlike mysql which only supports databases
* Great stored-procedure-language support
* NO LICENSE ISSUES!!! (for commercial products this is a god send)
* pluggable data-stores (gzip,csv,blackhole,commercial,custom)
* NoSQL back-doors (e.g. with INNODB)
* optional single-file consolidated data-store (INNODB) (postgres produces THOUSANDS of files which have a latency for file-open operations)
* linear append of "table-space" across raw partitions
* RDBMS "cheats" that help performance when you need it (can corrupt data if you don't know what you're doing)
* can store rows in primary key order (avoids 1 extra index)