From the World Heritage inscription:
Warsaw was deliberately annihilated in 1944 as a repression of the Polish resistance to the German occupation. The capital city was reduced to ruins with the intention of obliterating the centuries-old tradition of Polish statehood. The rebuilding of the historic city, 85% of which was destroyed, was the result of the determination of the inhabitants and the support of the whole nation. The reconstruction of the Old Town in its historic urban and architectural form was the manifestation of the care and attention taken to assure the survival of one of the most important testimonials of Polish culture. The city – the symbol of elective authority and tolerance, where the first democratic European constitution, the Constitution of 3 May 1791, was adopted – was rebuilt. The reconstruction included the holistic recreation of the urban plan, together with the Old Town Market, the town houses, the circuit of the city walls, as well as the Royal Castle and important religious buildings. The reconstruction of Warsaw’s historical centre was a major contributor to the changes in the doctrines related to urbanisation and conservation of urban development in most of the European countries after the destruction of World War II. Simultaneously, this example illustrates the effectiveness of conservation activities in the second half of the 20th Century, which permitted the integral reconstruction of the complex urban ensemble.
There are many European capital city centers which have been inscribed on the world heritage list: Paris, Rome, Budapest and Prague are just a few. All of those sites have been inscribed based on the history value of the buildings in the city.
Warsaw is different, however. This old city of Warsaw was almost totally annihilated during WWII. There is very little left which is of original historic importance.
Warsaw has been inscribed because of its reconstruction.
I’d be willing to bet that thousands of people visit the old city of Warsaw every year and have no idea that the buildings were all built after WWII. You have to really pay attention to see that the construction techniques used are very different than those you see in Prague (which was relatively untouched by WWII).
View my complete list of UNESCO World Heritage Sites.