My site is a turn-based games site. There are many others like it, most either charge for memberships or are free, advertising-based sites. Some seem to be very successful, some I would classify more as hobby sites.
A few of my current ideas are:
* Vacation days (a very standard feature... they extend the time limit on your games so you don't while you're away on vacation) (carrot)
* Advertising free (I don't have advertising now, so I'd have to introduce it for the free accounts) (carrot or stick?)
* 30-day free membership (stick -- don't like this idea)
* Limit on the number of simultaneous games a user can have (stick -- don't *really* like this idea, either)
* Prize tournaments for members only (need to research this idea... not sure I like it) (carrot)
And I'm considering a bunch of other minor perks (customize your profile, change colors, etc...) that would count as very small carrots.