Evangelicals think that the doctrine of Purgatory denies the sufficiency of Christ's Death on the Cross. The debate can get into one on merit, penance, as well as connection between the living and the dead within the Body of Christ. As well, the doctrine is "unbiblical". 2 Maccabees 12 implicitly mentions the existence of a third place (or state) where the dead goes, but because Protestants don't consider the book of Maccabees as being inspired, the current controversial topic may jump to another.
==> Purgatory (Catholic Encyclopedia)
==> Prayers for the Dead (Catholic Encyclopedia)
==> Doctrine of the Atonement (Catholic Encyclopedia)