From the World Heritage inscription:
Thebes, the city of the god Amon, was the capital of Egypt during the period of the Middle and New Kingdoms. With the temples and palaces at Karnak and Luxor, and the necropolises of the Valley of the Kings and the Valley of the Queens, Thebes is a striking testimony to Egyptian civilization at its height.
The Temples of Karnak and Luxor, together with the Valley of the kings and Queens are some of the most impressive sights in all of Egypt. You can read more about Karnak in my Seven Wonders of Egypt.