The issue I see the most is that NoSQL has become synonymous with distributed databases which is really where making ACID promises becomes a challenge. If you had a single node document DB, it probably wouldn't be very challenging to make it ACID. SQL or not, as soon as you want to make it automatically shard and distributed, then you will suffer large performance impacts with transactions. Are there any single node document DBs which provide ACID guarantees out there? I know Redis can do it with multi/exec but Redis isn't really document oriented though it's an awesome piece of software.