I think what happened here is that a subtle debate about whether the characteristics of relational databases are a good fit for web development* has been hyped

I think what happened here is that a subtle debate about whether the characteristics of relational databases are a good fit for web development* has been hyped into some kind of movement. It doesn't help that a many peoples' first impressions of RDBMSs these days probably involve reading just enough of a MySQL tutorial to use it for a PHP project. Working with relational databases without understanding the relational model (e.g., understanding indexing, the rationale for normalization, good schema design, etc.), can lead to disappointing performance, and in those circumstances, people are unlikely to have someone better-informed convince them otherwise. (The object/relational mismatch is part of it, as well.) * Among other things, RDBMSs put a lot of resources into ACID transactions -- in the use cases for which they were designed, data is expected to outlive some employees, and just dropping part of an update to e.g. a medical record or insurance claim would be a disaster. Similarly, the databases can do elaborate checks on the data, and any potential changes, to ensure that it's always internally consistent. Losing a youtube comment wouldn't be a big deal, though, and if you're willing to cut corners on transaction guarantees or internal consistency, there are performance benefits. (Being extremely protective of the data is a good default, IMHO.) And, yes, not just relational DMBSs are ACID, but many "NoSQL" dbs have that trade-off in mind.