I think that Stefan-Boltzmann does constitute a negative feedback, as an increase in the surface temperature, will result in an increase in outgoing radiation. Other negative feedbacks include the carbon sinks; plant absorption and silicate weathering. Any spelunker can attest to silicate weathering in the form of limestone deposits, although this is a very slow process of removing CO2 from the atmosphere. Left alone, plants would remove CO2 at a much faster rate. However, deforestation has been working against this feedback for centuries.
The bigger, faster negative feedback is clouds. We have already seen this in the Arctic, as increased couldiness has warmed the Arctic in the winter, and cooled it in the summer (although this is still speculative). Clouds have always been a big contention and question mark in the entire equation. Clouds do reduce heat loss, due largely to the water vapor contained within. This is overpowered by the albedo effect, reflecting more incoming sunlight. Whereas the net effect of increased atmospheric water vapor is to warm the surface, the net effect of increased cloudiness is to cool the surface. This has the potential to be the biggest negative feedback loop.