Overall I enjoyed reading the article and the specific tradeoffs you had to consider when comparing Riak to Mongo for your specific use cases. I'm curious if you had any problems w/r/t the 3 items you highlighted above when using Mongo, as none of those were mentioned in the linked article.

(these points are for MongoDB 2.0+)

1. Adding a new shard in Mongo will cause the data to be automatically rebalanced in the background. No application-level intervention required.

2. Node failure (primary in a RS) is handled without intervention by the rest of the nodes in the cluster. Network partitions can be handled depending on what the value set for 'w' is (in practice it's not an isolated problem with a specific solution).

3. Using mongos abstracts the need to know what node any data is on as each mongos keeps an updated hash of the shard keys. Queries will be sent directly to the node with the requested data.