Basically they take everything and give nothing. If you don't understand the terms of the agreement, you absolutely shouldn't be signing it. You know, the whole bit about when you can't spot the sucker at the table, etc.
a) they take 80% commission instead of 30-50%
b) they demand exclusivity and a whole bunch of other rights you wouldn't normally hand over
Some articles: JMG, Hawk and Thornton.