It all comes down to what you mean by rant. On off-topic stuff, that is what the BCS is for, as JPC said. It's a free range sort of thing, so if you want to post on knitting and needlepoint, go nuts. But also realize that if there aren't others who share your interest, your thread will probably just sink.
But if your purpose is basically just to whine, there's no place for that. There are plenty of forums on the internet to rant, write a blog, go to Facebook, use Flickr on random images like some people do. Harsh? A little, but that's the nature of the site we've created, and the community we have. So ask yourself, what would it really add to that community?
You know, if I see links to Brickshelf, more often than not, the first thing I click is the "Back" button.