I LOVE categories (ie. on Wikipedia) that can have more than just one hierarchical parent. (ie. Schwarzenegger parents: Actors, Governors, Mr. Universes, etc.) Top categories like science would have a lot of sub-fields, many that would overlap with other fields like medicine, engineering, space, etc.

I really like the idea that you sub to all children-subs too. It simply makes sense. I would unsub from games and most sports.

These "subs" can simply be categories or metatags too, if that makes it easier.

Important on the wikis: do not loop categories because apparently the server keeps needlessly processing. (ie: Don't make your grandfather your child.)

Easy simple incentives are good.