I'm always reluctant about going completely schemaless. Reminds me of the blog post by FriendFeed about how they use MySQL, highly recommended: http://bret.appspot.com/entry/how-friendfeed-uses-mysql

I feel like like there needs to be a better middleground for having some schema but being able to augment it easily with metadata that you're not querying against (yet). Then later extracting the metadata into queryable columns.

I wrote a post outlining some ideas of how to do this: https://github.com/shazow/everything/blob/master/idea/arbitr...

I've only implemented bits and pieces of this in practice, huge convenience so far.