Just off the top of my head: - First class support for sharding & replication - Pluggable storage engine - Capped collection (i.e. tailable table/collection)

Just off the top of my head: - First class support for sharding & replication - Pluggable storage engine - Capped collection (i.e. tailable table/collection) - A query language which doesn't boil down to string concatenation (easier to manipulate in a language of your choice, but harder for human friendly adhoc query writing) Not that postgresql doesn't offer any of the above in some form or another. Just that MongoDB does seem to make 1 & 2 & 3 easy to deploy & manage