From the World Heritage inscription:
Sukhothai was the capital of the first Kingdom of Siam in the 13th and 14th centuries. It has a number of fine monuments, illustrating the beginnings of Thai architecture. The great civilization which evolved in the Kingdom of Sukhothai absorbed numerous influences and ancient local traditions; the rapid assimilation of all these elements forged what is known as the ‘Sukhothai style’.
Sukhothai is what I wish Angkor would become. While not nearly as large as Angkor, the grounds of Sukhothai is still very large, I’d estimate about the size of Central Park in New York. It is also cared for like a park. The grass is trimmed, the roads are in good shape, there are paved walking paths, and everyone just looks nice. They also offer bikes for going to the various temples, which is a very popular option.
It is one of the least visited tourist attractions I’ve been to in Thailand. It is about midway between Chaing Mai and Bangkok. I’d highly recommend it as a stop if you are going from Bangkok to the north.