You should check out multi-document ACID transactions on MongoDB 4.x. (Yes things have dramatically changed... add in change streams and JSON schema validation and $lookup and $graphlookup in aggregation)... (And if you don't want to install it locally, try a free M0 on MongoDB Atlas to play with the new features).
And yes, I work at MongoDB, and I worked with it before and it has really changed - WiredTiger has been a game changer for it, which is why I work at MongoDB now.
Dj from MongoDB here. We have, obviously, been keeping up with this and other threads, but we've also been busy testing out Amazon DocumentDB's correctness and performance. While we're getting that together to bring you an official response in a few days, complete with test results and methodology, I'd like to pick up on a couple of points and some inaccuracies that have been repeated in various threads:
This move shows MongoDB’s approach to document databases is compelling. We’ve thought so for a long time.
A cloud-hosted, truly global and managed MongoDB, MongoDB Atlas, has existed for the last two and a half years and has been serving more and more satisfied users every day with some massive workloads.
MongoDB Atlas runs the full implementation of MongoDB in the cloud.
Many features of MongoDB are documented as not being implemented by DocumentDB: these include change streams, many aggregation operators including $lookup and $graphlookup. But beyond that, well let’s just say we’ve been staggered by how many tests DocumentDB has failed (no spoilers!).
The MongoDB API is not under an Apache license.
MongoDB drivers are still under the Apache license. The MongoDB server used to be licensed under AGPL and is now licensed under SSPL. The source code is open to all, as it has always been, at https://github.com/mongodb/mongo
DocumentDB is not cheaper than MongoDB Atlas. Preliminary estimates show this to only be the case with very large collections and very, very high read/write workloads.
There’ll be more next week over on the MongoDB blogs.