I do enjoy building cities. To me building or exploring a city is the more important part of the game than simply playing one family to get their goals - I do play that way sometimes, but it's not as satisfying.
Also I have lots of Sims whose stories I've started to tell, before the cities/worlds they lived in became corrupt or impossible to play.
While I would LIKE to play a 10-generation dynasty, I've never had a game last that long and still be playable, though it's easier to move a family from one world to another now.
Some comes from Sandy's lovely expose/tutorial on building a city.
I don't get on with Create-A-World, so I have to use other people's worlds. And on the whole that's fine. I have downloaded "fixed" versions of most of the EA worlds, plus some marvellous worlds made by Simmers.