I may be biased because I work on it, but I think TokuMX solves or will soon solve all of the "big data scaling" problems that exist in MongoDB. There is defin

I may be biased because I work on it, but I think TokuMX solves or will soon solve all of the "big data scaling" problems that exist in MongoDB. There is definitely still room for polyglot strategies: Zookeeper is for tiny but crucially consistent data, Riak is for fancy distributed systems availability guarantees when you can afford a simple data model, and I believe Redis has value as a sophisticated programming model for things you can fit in RAM (but I actually have no Redis experience personally). But in the past few months, I've grown to be really impressed with the document model, and the aggregation framework is getting more and more powerful with each release. I think that's the important part of MongoDB (sharding's a little messy and Riak seems to have their heads on straighter with that), and TokuMX takes that and sands off the rough edges you see with big data sets and concurrency, and I think that's going to end up dominating MongoDB and being a really compelling point in the NoSQL space.