[mrinterweb]

I would say that document-level locking is a big deal for MongoDB. When I was working at a company running MongoDB, we were running into latency issues when multiple larger documents would be updated or created. Coupled with the fact that we were using AWS and provisioning even 1000 IOPS was expensive and limited compared to IOPS available to SSDs. On top of all of that we weren't using the fire-and-forget default for writing data. We were requesting write receipts.

Document-level locking will likely result in much better performance for MongoDB.