For those sufficiently interested, the below may make enjoyable reading:

http://www.databasecolumn.com/2008/01/mapreduce-a-major-step-back.html
http://www.databasecolumn.com/2008/01/mapreduce-continued.html

(both write-ups are about one and a half years old).

In the general case, Jim's observation on transactions makes updates hard and Micheal's observation on joins makes retrieving selected data hard. Both kinda essential for a general purpose database.

Paul