One can send out only five scrolls at a time
One can receive only up to three scrolls
Only players that haven't played World of Warcraft for at least 90 days are eligible to receive scrolls.
One can accept only one scroll per account
Scrolls were previously restricted to use on accounts with no expansions, but now the effects have been adjusted so that anyone will get 10 free days of access, concurrent with a trial of the next expansion after whatever they have installed (unless they already have all expansions installed, including Cataclysm, in which case they only get the trial time)