I've been using DynamoDB since it was released. I wrote a nodejs driver[1] and a nodejs data mapper for it[2], so I have a decent bit of experience with it. Browsing the DocumentDB docs the two services seem to be very different. DynamoDB is really just a key value store with some very nice properties, but also a lot of tradeoffs. One such tradeoff is querying data is very limited in DynamoDB. In Dynamo you can only query data by its primary hash key and optional range key. These keys you must specify upfront when you create your table and cannot be changed afterwards.

DocumentDB seems much more similar to mongodb and appears to have a very flexible query ability. In my opinion, one of the best features with DynamoDB is you can tune the number of reads/writes each individual table requires. This lets you scale up and down your database and greatly helps keep costs down. This is a feature that only a hosted database service can offer. I haven't yet read any pricing info on DocumentDB, but hopefully they offer a similar feature as this is really where a hosted database service can shine.

[1]https://github.com/Wantworthy/dynode [2]https://github.com/ryanfitz/vogels

DynamoDB really isn't document oriented storage.

Instead it's a key-value wide column store, with 1st and 2nd level index support.

I haven't delved into the MSFT pricing model, but DynamoDB is pay-for-through put. You provision your table with a certain amount of read/write performance that is guaranteed, and you pay for that.

This allows you to use SQL queries, and built in index support