You have Karma, mojo, XP, etc. Wikipedia is an organic meritocracy. When it comes down to it, there are only two metrics on the web that make sense.
- Time. Time devoted to the site, online presence, GTKY, etc.
- Money. Subscriptions etc.
Money is difficult. People who subscribe to get special features creates separation. Ads get blocked, and reduce coherence. I think the solution is found in what the subscription represents. It says "I value the site such that I give $xx a month to make sure it stays around". In exchange for that, the site should value them right back, acknowledging the compliment and returning it somehow, for example listening to their feature requests, feedback etc. The subscription shares some features with owning shares in the site, just in reverse. The dividend the user gets from the site is what made him subscribe.