I have played other MUD style games, from your basic "kill everything" style to ones that are based on a specific book series. TriadCity is a wonderful game because it mixes just the right amount of both, with real-life satire mixed in for good measure.
For someone like me, who enjoys exploring,(woods, cities, anyplace that calls out to me really) TriadCity fits the bill because it emphasises exploration. But even more than that, it emphasizes thinking while you are exploring. I have not found a game yet that I can be fully content with just wandering around and reading the world around me.
When I am done with all that, I can emerge myself in the arts of the game. From being able to write and article for the newspaper, submitting a painting for the Royal Library to display in-game to directly helping build the world with the Builder Role.
Sure there is combat, theft and all the fun things you would expect any game to have, but for me TC is about the ability to directly interact and change the culture of the city. That is why I will always go back to TC. Where else can you have such a direct interaction with a gaming world?