Re: go Postgres, go
The issue is Oracle is not just a SQL processor you throw some SQL at. Oracle is a big system from ETL to backup/recovery of large databases - in a single product. In the middle, you have advanced features for access control, data storage (it supports its own dedicated file system) and handling, clustering, replication, development, ecc. ecc.
Lots of features you may not use in your average DB behind a "simple" web site, but becomes more and more important when the database stores lots of accounting, financial and sales data - or any other kind critical to your business, and many different applications and users access it.
Till now, most open source database technologies didn't try to compete directly with the high-end databases, focusing instead on eroding a growing slice of their lower- to middle-end customers. NoSQL database are another league, they can handle big tasks but requires different applications and a very different way to manage data, and are less versatile.
Sure, today you'd probably be a fool if you used Oracle when other solutions perfectly fit, just there are some areas where other solution don't fit so well, and Oracle knows and exploits this to ask a lot of money, because competition there is very small - and MS or IBM are not exactly loved as well...
Biting the hand that feeds IT
