What makes halal slaughter really halal? If you include scientific proof, more plus point for me.
Because Halal slaughter is intended to inflict the least pain on the animal by cutting the arteries which suddenly drains the brain from blood and renders the animal brain-dead and thus unable to feel the pain any longer. Saying "Bismillah" and facing the animal towards Qibla are also meant to highlight the awareness that the slaughter is a gift of God and therefore done with observance of His oversight and moderation, thus sanctifying what could be an otherwise unrestrained greedy exploitation of animals.
See Wikipedia entry Dhabihah for a more comprehensive explanation and also modern questions in regards with legitimate slaughter.