what is the key difference comparing to wordpress.com
Neocities sites are static HTML, meaning once your files are uploaded,they don't change. Neocities doesn't do POST, there is no database backend or code running on the server.
Now in 2017, CORS allows client side apps to do just about anything traditional web apps have done, without page tranistions, given an appropriate backend. The difference is that html rendered by javascript on the client typically isn't indexed by search engines, while html served from a domain are. That's one significant difference, there are others.