From the World Heritage inscription:
With its strategic location on the Bosphorus peninsula between the Balkans and Anatolia, the Black Sea and the Mediterranean, Istanbul has been associated with major political, religious and artistic events for more than 2,000 years. Its masterpieces include the ancient Hippodrome of Constantine, the 6th-century Hagia Sophia and the 16th-century Süleymaniye Mosque, all now under threat from population pressure, industrial pollution and uncontrolled urbanization.
I don’t even know where to begin talking about Istanbul. It has quickly become one of my favorite cities in the world. There are several world heritage sites (Rome, Paris, Prague) where everything in the city is lumped together because there could be multiple sites which would merit listing otherwise. Istanbul is one of those cities. The Hagia Sophia, the Blue Mosque, the Topkapi Palace and many other places would justify inclusion as world heritage sites on their own merits. Istanbul is truly one of the great cities of the world.
View my complete list of UNESCO World Heritage Sites.