The colour. Egyptian temples are stone that has lost its paint; these tombs kept theirs. Ceilings of deep blue with yellow stars, walls of figures in red and green and white, hieroglyphs as crisp as the day they were cut. Nothing in the open-air sites prepares you for it.
The engineering is its own subject. These are corridors driven hundreds of metres into solid rock with copper tools, descending on a planned axis, with pillared halls and burial chambers at the end. The tomb of Seti I is the longest and finest of them, and if it is open and you can afford the supplement, it is the single most impressive interior in Egypt.
And there is Tutankhamun. His tomb is small, plain and unremarkable by the standards of the valley — it was not built for a king who would die at nineteen — but his mummy is still there, which is not true of any other pharaoh in the valley.