The thing I noticed about NoSQL is that, at least with Mongo, you just end up putting your schema in your app. You probably do it by saying a particular class has a bunch of fields or instance variables that you expect to be a certain type.

I wrote a portion of an app that relies on Redis, and despite the fact that it's really nice to work with, especially in Ruby (that practically mimics Redis' native API), it was incredibly difficult to remember the structure. What keys do I need? What type are they? Oh I'll have to do `KEYS *` and look I guess. Then I'll remember to document it (and thus create the schema).

Of course, the problem on my part was, in part, that I probably didn't need to use Redis. And when I needed to use Mongo, well, I wasn't storing things that look like documents. I think this is an easy mistake to make, but one you learn from quite easily once you've experienced it yourself.