One particular part of that slide deck stuck out, and b/c there's no accompanying audio to go with it I thought I'd mention it. Towards the beginning of the deck in one slide he says 'MongoDB is fast.... until your data / indexes no longer fit in memory'. Later on there's the '120GB of data in MongoDB became 70GB of data in Postgres'. The next slide talks about using short key names + a key-mapping layer to preserve space.

While using longer key names in your objects will lead to larger objects / larger storage requirements, indexes do not contain the names of the keys of the documents they are created on, so if I have 10M documents in a collection and an index on a field 'ts', that index has the same size as if the field were named 'timestamp'. It's only the storage of the object that grows in size, and by the time you are handling 120GB of data it's very reasonable to imagine you would be using Memcached in front of MongoDB to obtain the actual objects.