The irony of this is that it's actually quite easy to build a document store on top of PostgreSQL that performs very well indeed. I tried it. I used a similar approach to the one described at [1]. You get the advantage of years of experience (never underestimate this!), MVCC, transactions, consistency and replication as well.

(yes I know it doesn't "perform" as well as the other NoSQL stores but performance is not without tradeoffs [2])

[1] http://bret.appspot.com/entry/how-friendfeed-uses-mysql

[2] http://www.caradvice.com.au/wp-content/uploads/2006/12/Ferra...