2:1 ratio: you have to send out at least 1 book for every 2 you receive. If you don't keep this ratio up, you won't be able to mooch any books, even if you have the points, until you improve your ratio. Sending internationally counts as 3 books.
Feedback score: if your feedback score is bad, people may refuse your mooch requests. Efficient response and well-packaged books in good condition earn good feedback points.
Lost in the mail: if a book is "lost in the mail" the sender gets their point anyway, and the receiver doesn't lose a point. You're allowed a limited number of "books lost in the mail" in order for us to limit possible fraud.