It really was the first NoSQL database that many programmers used -- and it came out right around the same time that JSON was really taking off. The timing was perfect, the product was simple. Fact is most people don't really need insane scale and there are thousands of use cases where you just need to read and write JSON and fetch things by a few different fields. I think they nailed it. Every other NoSQL DB out there at the time was insanely hard to "just play with" (e.g HBase, Cassandra etc.)

I think you hit the nail on the head -- for many programmers who were "coming of age" at that time Mongo was the easiest to experiment with and to get working.