> It's sounds really nice to have a data store that adapts when it's impossible to know up-front what you need from your data structures.

This seems backwards to me. Relational databases are much better at ad-hoc querying of data, whereas NoSQL scales for narrower access patterns. The fact that you can dump arbitrary columns without a migration is a nice convenience, but in general it will be less queryable than it would be if you added the column in a SQL database.