Unfortunately both are useless for mortals because there is no transactions and locks. It's just impossible to write applications where uniqueness and real ACID transactions are required. Please don't send me to CouchDB bulk document API, I don't wanna dance with conflicts and inconsistency every time I am going to write simple application with user registration for example. The same for apps with direct purchases where you need to update quantity of item in stock. Just impossible. Solutions with "inventory tickets" sounds insane. Unique fields as an _id also sounds insane, because it's impossible to create complex "unique keys".
Both are cool, seriously, and you know. Lots of developers are going to use this software, but every time the lack of such features just rejecting these people.
1. Add real transactions to CouchDB/Mongo 2. Add unique indexes to CouchDB. IIRC Mongo has already. 3. Add map/reduce chaining to CouchDB 4. Dominate the world!