I've found MongoDB to be the perfect datastore for early-stage startups (which are presumably evolving rapidly) for exactly the reasons Mike mentions:

- it's super flexible (and schema-less)

- dynamic queries are fast and powerful

- it's very easy work with

Scalability is another issue, but I think Mongo can scale (and that's slowly and bravely being proven by Foursquare). And almost all startups need to drastically re-architect their datastores when they hit massive scale anyways, so why not start out with the easiest and most flexible option that will let you iterate the fastest?