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])
To me, the biggest advantage of MongoDB is autosharding. If you read the friendfeed article, you'll see that they also have to deal with 'eventual consistency' on the indexes.
I'd say that's not necessarily an advantage. Sharding is incredibly complicated to get right considering all factors such as balancing and recovery.
I'd rather partition the data based on function onto distinct clusters, you know like eBay do.
You can update the indexes in a transaction too, so that's not necessarily an issue. MySQL has problems with this due to locking but the MVCC implementation in PostgreSQL allows much better concurrency.
