I wouldn't call it dead space, and it shouldn't be deleted. Just because it isn't currently active doesn't mean that people don't view the information there. After I finish GT3 I'm going to borrow the original GT and GT2 from a friend. I'll need to look up information on those games and GTPlanet will be my first choice. If you're deleting inactive threads/forums, why not delete all the closed forums as well? Because the information in them is still relevent, even though no one is adding to it. I mean, might as well toss the Bible while you're at it! (:
~LoudMusic