Invoices seem like a pretty good candidate for a document DB to me.  It's much, much easier to store a list of "line items" in a single document than it is to extract those out into a properly normalized structure.  What advantage does having that data across multiple tables give you?<p>Blog comments, on the other hand, might benefit from more normalization.  Cramming a bunch of comments into a post document could result in contention problems, depending on the engine and how you're handling them.  In CouchDB, you'd end up having to update the entire post document for each new/changed comment.  Mongo handles it better, since you can atomically push new comments into your doc.  Still, I think I'd rather have them in their own documents on a busy site.