On the other hand, if you abstract your interaction with data enough, you can change the whole backend later once it's stable and not care about it up-front. T

On the other hand, if you abstract your interaction with data enough, you can change the whole backend later once it's stable and not care about it up-front. This is a conception of data that is more true in theory than it is in practice. In practice, if you want to query your data and efficiently, you'll need to worry about how it's stored. You'll have to worry about the failure cases. Of course, it is definitely application dependent. If you're just writing a Wordpress-replacement, you can probably choose whatever data store you want and just write an abstraction layer on top of it (especially if you don't care about performance). On the other hand, if you're looking at querying and indexing terabytes (or more) of data, you'll have to work very closely with your data store to extract maximum performance.