Same thought and it being at an early stage, ugh. And there goes at least a dozen of competitors out there trying to be different than MongoDB. I am just sort of happy that in the SQL world we usually either look at MySQL or PostgreSQL (well, Oracle and SQL servers are probably more relevant to corporate web service)... but I think people are trying to migrate too.
I think that even in a NoSQL driven domain, that a classic SQL based RDBMS has a place. It's that certain types of load have acceptable levels of relaxed constraints.. that can increase when your data is searched/read over 1000 times for every write. That joins are expensive and even mirroring data to a nosql store has benefits over purely rdbms.
I like document stores like MongoDB and RethinkDB and feel they are a great fit for most scenarios. I also feel that caching layers with Redis or Memcached can help...
Cassandra is interesting in the primary storage space as well, and imho has resolved a lot of issues, while others remain. I'm interested to see if this database can get there faster than Cassandra/CQL can get to more parity with traditional SQL systems.
While I appreciate the options, there is no one solution for everything... If you never break 100 simultaneous users, memory-mapped flat files and map/reduce could be sufficient.