In addition to the factors mentioned, some very interesting researchers has also found that giving people compensation for their work can actually decrease their motivation for performing a task.
from http://www.newsforge.com/
Quote
What happens when a free and open source software (FOSS) project attempts to introduce compensation for its developers? Because FOSS remains based largely on volunteer work, many worry that payment might demotivate both those who receive it and those who do not. However, community leaders who have observed how payment interacts with the FOSS ethos suggest a more complicated picture. Identifying four main types of payment -- bounties, payment in kind, grants, and employment -- these experts suggest that what happens depends on the type of payment, as well as on the individuals involved...
from http://www.newsforge.com/