I don't know about "perfect", it certainly can fit the relational mold though.

I'll take a shot in the dark and say that the advantage of using a NoSQL platform to host a forum is that you can very quickly load all resources relating to a post by storing them in one database document.

That does mean you probably need to polyfill certain functionality, but the traditional case for NoSQL is made by saying a blog can load a given post's comments, media, etc. with one query. Forums are a similar animal.