Currently when you Edit a set, you can set "Item Attributes". These appear to all be minifig related attributes. Shouldn't sets have their own set of attributes? I'm not sure what objective attributes could be applied that aren't covered elsewhere. Perhaps TLG's recommended age range (e.g., 6868 Hulk's Helicarrier Breakout would be 7-14). Piece count seems to be missing from the Set attributes as well.
I suppose you could also have something like:
Main Subject: Building, Vehicle, Scenery, Minifig, Game, Robotics, Playbrick, Train
Scale: Minifig, Micro, Sculpture
Similar story on Instructions, Packaging, and Stickers. They currently have minifig related metadata. I could see Packaging have a Packaging Type - Box, Polybag, Foilbag, Plastic...