As one of several game makers on HN, I want to take issue with the idea that "games are essentially just user interface." Games almost always present an interface to an underlying system. Many games have an underlying system that is not particularly simple or intuitive and take significant effort to learn. And, because unlike tools, they do not offer to assist the user in doing something that she already knows she wants to do, we often provide many more incentives along the way, as struggling with a system is often not amusing, and amusement is usually the goal.
This is all to say that the UI is very important, but to consider games as chiefly UI is to fundamentally misunderstand what games are and how we interact with them.
Yeah, my statement's only true in certain senses. It might be more accurate, with tabletop games at least, to say that games are a UI to the other players - that is, they channel social interactions in certain ways.
this is my feeling. the games that last are ones that distill some aspect of friendly competition between people. and the very best games are the ones that cause you to interact with your friends in some new way (games that involve auctions or mini-economies tend to do this well)
This is all to say that the UI is very important, but to consider games as chiefly UI is to fundamentally misunderstand what games are and how we interact with them.