For me, it was a database schema with 38 joins (and 2 additional queries) to effectively get the data to display a single page. For that use case, mirroring the data on save to MongoDB was a no-brainer... with geospacial queries out of the box, and a few other indexing features it made a lot of sense.

I wouldn't even think to use MongoDB for certain use cases... but for others, it's a great fit. I think that Cassandra, Riak, Couch, Redis and RethinkDB all have serious advantages and disadvantages to eachother and SQL.

I do find that MongoDB is a very natural fit for node.js development, but am not shy about using the right tool for a job.

Another thing that tends to irk me, is when people use SQL in place of an MQ server.