I also think flat file storage is a valid solution. If you are abke to use advanced filesystem features like snapshots and online replications, the use-cases for databases shrink even further.
You can get repository level transactions by committing on each consistent state (squashing before push) this lets you do undo/redo and recover from failed operations easily - databases sort of give you this - but only for the stuff they actually contain (so unless you're putting your asset files in to the DB you don't get this).
"Server-boundary crossing edge-cases and race conditions persisted for a long time -- allowing for gold and items to be duplicated, though I think we had largely eradicated the big ones by the time I had moved on to UO2."