Pruning attachments makes sense, since I'm positive there will be many duplicates. I'm not sure about old topics, but that's just me protecting my beloved GD. Maybe older topics from Off-Topic and the Birthdays Thread, clean up Necrum etc.
Also, you could get rid of all those unwanted members that never post. You always refute such a task, but you can't deny it would save
some space. :P
PS - there are five winking Fuzzle Oddicons to my left, and two green faces sticking their tongues out at me.