Agreed, the comparison of Mongo to databases as Rails is to frameworks is appropriate. I recently started playing with Mongo in a Rails app and the Array type led to an a-ha moment followed by giddy handclapping and a fair amount of code deletion.

Working at a large company that's pretty invested in RDBMS, I'm curious to see where the downside of using something like MongoDB comes in. E.g. James Golick's creation of Friendly because they hit a scaling wall — how big do you have to get for it to become an issue?