[chrispadfield]

well your first decision is is the subscriber status going to be made by using a usergroup (in my opinion a bad plan because users can only be in 1 usergroup) or by an extra field in the user table (IMHO a good idea).

Benefits include:

1) Seperate header/footer/headinclude templates (to take away banners and pop-ups
2) Different pm storage limits
3) Different mass send to buddies limits
4) Avatars only for subscribers
5) Different post settings on custom avatars
6) Ability to post public events in calendar
7) A forum for subscribers only
8) Turn off limits such as 60 seconds between pm/searching etc.
9) Recognition icon.
10) A subscriber page
11) Automatic email notification of subscription expiration.

probably more....