There's a lot of complicated financial balancing possible. Every hot dog stand can have its prices adjusted, you pay for every piece of shrubbery and maintenance worker time and you set admission fees and conservation priorities and it seems endless. Financial balancing is not really my idea of fun, especially not since I got my most recent dental bill. I have decided I am going to be a very rich zoo tycoon, since I can set the amount of money I begin the game with. I will throw money at all my problems and have a ton of animals. Now, that's a decent fantasy game.
One thing I like is the weird way time passes; you don't have to watch things day by day, but months drift by as you play. You don't have to pay attention to feeding and cleaning up after the animals on a daily basis, so you're freed from the monotony of Sims-dom where it seems like you spend the whole damn game making people remember to shower and eat and pee (and, after installing Living Large, clean up their cockroaches twice a day. I think that's what really helped me kick the Sims habit).
I haven't seriously built a zoo yet -- didn't save the game after the lion ate the zebras. But it looks like fun.