Not stupid, but there are different perspectives that reduce the scariness. With are relational model, your working set needs to be joined together from pieces

Not stupid, but there are different perspectives that reduce the scariness. With are relational model, your working set needs to be joined together from pieces, so you want ACID to ensure that everyone sees a consistent set of pieces. But with a document model, you can 'pre-join' your working set into a single object that has everything you need. And that object doesn't have a fixed schema, so it can grow and evolve over time. While Mongo doesn't provide ACID across documents, it does guarantee atomicity, consistency, isolation and eventual durability for SINGLE documents. IF your application can live with a universe that consists of a single, arbitrarily complex object - then Mongo is as within epsilon of being as safe as a regular ACID transaction system.