I'm just learning to use MongoDB, and when discussing with other programmers would like a quick example of why NoSQL can be a good choice compared to a traditional RDBMS - however the scenarios I come up with and can find online seem pretty contrived.

E.g. a blog with lots of traffic could be represented relationally, but will require some performance tuning and joins across tables (assuming full denormalization is being used). Whereas MongoDB would allow direct retrieval from one collection to the same effect.

But the response I'm getting from other programmers is "why not just keep it relational and then add some trivial caching later?"

Does anybody have a less contrived example where MongoDB will really shine and a relational db will fall over much quicker? The smaller the project/system the better, because it leaves less room for disagreement.

Something along the lines of the complexity of the blog example would be really useful.

Thanks.