Buffy didn't exactly die, in Prophecy Girl she stopped breathing for a minute at most.
Death is the total cessation of all vital organs, fact is, after a minute she would still have had brain activity and a heart beat, if very faint.
IF she had 'died' then no amount of CPR would have revived her, infact CPR, contrary to popular TV belief, rarely revives someone, its a method to keep someone breathing and blood pumping until the proper equipment can be brought in to revive.
So, the reason Buffy didnt actually go to Heaven, was because, although the Slayer Line assumed she had died and passed the line on, the Soul was still encased in the dying Body waiting for its release.