Economy sounds too similar to rep. Rep is basically the same thing, only there's practically infinite amount of it (whee inflation!)
Each time a newer member joins there will be more economy (whee inflation!)
I dunno. I've been on forums where there was an economy system where you could buy colored titles, username changes, user text changes, changing other people's text (for a price)...etc
Personally, I feel way too much of a hassle.