One important factor which in certain contexts will have a higher impact than extrinsic rewards is surely the intrinsic rewards. This is in particular true in competitive and reputation-based environments, where recognition for completeted achievements is importantant for differentiations among peers. Enabling people to be perceived as "experts", "gurus" etc, can be enough of a driver to influence engagement patterns, and inspire people to submit content, actions etc.
Other intrinsic rewards which can influence user behaviour is learning as well as entertainment. Such elements can equally create a drive for visitors to see it being valuable to participate.
Extrinsic rewards such as discounts, gifts etc surely have a value as such, but will probably not generate a complete change in behaviour, driven by differentiation in the way that intrinsic rewards can do.
