The improved performance numbers are mind-boggling. Write performance improved an order of magnitude? Up to 95% less overhead?

Does this mean the previous versions weren't very good, or have they done something exceptionally smart?

I believe the former one is more likely. Wen MongoDB was compared against other databases it was failing in pretty much every metric.

Release notes: http://docs.mongodb.org/v3.0/release-notes/3.0/

Still no joins...sad. Well, back to ArangoDB :).