One of the main sticking point for serious mongodb infrastructure is the global lock -- yes, write operations cause the entire mongodb instance (every database, every read operation) to block. This is clearly one of the main issues surrounding high-concurrent mongodb development.

Thankfully with mongodb-2.2 there is now instead a database lock which mitigates the issue.

Personally I feel redis more closely resembles language primitives with simple key-value pairs, hashes, lists (ordered, repeating values allowed), sets (un-ordered, no repeating values), and sorted sets (more interesting).