What are favorite features of MongoDB?

Aggregation framework. Very stepwise, and easy to understand.

There seem to be real-time features as well.

What MongoDB is missing are,

• $lookup / JOIN performance
• Complex aggregation performance, sometimes. Not sure if RDBMS would do it better.

Foreign key constraint is also missing, but I don't think it is important for me.

I found a use case to replace a Postgres database of open data sources I was maintaining for my job. The data was being pulled from a few sources into Mongo, but it took a lot of time to process and create tables and load the data where I wanted it. I switched to mongo, mostly using the PyMongo API. It took me an afternoon to watch some videos get it installed, and rewrite a half dozen pipelines to load the data into MongoDB.

I loved that I was able to learn enough to get what I needed out of it in a few hours, and haven’t needed to go back into it since. I am also able to add new data sources faster than I used to, so I am way more productive.

c# guy here. coming from a sql+entity framework background, the learning curve for mongodb was massive for me. but I'm never going back to that god-aweful tables+columns+rows nightmare again. my apps don't need an ORM layer anymore. thinking in objects/entities all the way down to the DB is a huge convenience. it's super fast for most use cases. the aggregation framework can run circles around sql queries imho. sharding/ replicating is easy enough. i developed my own library on top of the official mongo driver simplifying the API and adding some features such as migrations, fuzzy text search, gridfs alternative, easy relationships, etc. I've written a few articles about those here on DEV if anyone's interested.

I like it cause it works well with Node/Express

Streams. If you will read about it in official docs, you might understand one piece of what makes YouTube so performant. Also will make tou understand some secret concepts of how some S3 stuff works.