From the World Heritage inscription:
The Gyeongju Historic Areas contain a remarkable concentration of outstanding examples of Korean Buddhist art, in the form of sculptures, reliefs, pagodas, and the remains of temples and palaces from the flowering, in particular between the 7th and 10th centuries, of this form of unique artistic expression.
Historically, Gyeongju should be on a par with Kyoto or Nara. Gyeongju was a formal capital of Korea. Unfortunately, time hasn’t been so kind to the temples and palaces of Gyeongju as it has been to the former capitals of Japan. Centuries of war have left few intact historic structures.
In addition to a top notch museum of Korean history (one of the best I’ve seen on my trip) the most significant features in the city are the burial mounds. You can find them all over town, including the burial mounds of kings of the Silla Dynasty.
Also within short driving distance of Gyeongju is the Seokguram Grotto and Bulguksa Temple World Heritage Site.