Well, real streaming queries, not just oplog tails for starters... a document database with the ability to do on-server joins. There's a few other things, but those are the two biggest ones over most others in the same mold... All that applies to Mongo, applies to rethink.

Oh, yeah... and the ability to do shard + redundancy, where mongo you're either or have to do both. So scaling works a bit better. The admin tooling for rethink is better than anything I've really used in non-sql databases. And frankly even better than db admin tools, including sql based ones.

It's just a really great, stable database with really good scaling for the majority of use cases.