Gameplay resembles SimCity, in the sense that you're working to continually balance a series of systems to keep things moving, but it's different in the sense that you're not building anything. You are balancing, tweaking, and adjusting systems which leads to ask questions about how your changes impact the planet.
It's SimCity style controls and those are pretty intiutive. Menus, sliders, and pointers. What else could you possibly need.
There is no story that I could see, but that's expected considering it's a simulation game. The reason it's good enough for the game though is that it's expected and you can tell your own stories with systems at hand.
I'm sure there is an argument to say that you can make your own puzzles to try and figure out how to form a planet that has specific characteristics, but early on in the game I didn't quite understand the point other than to run the simulation. The puzzle really became figuring out if the dial or slider I used to change something actually did something.
Co-op Style: None
It's a simulator. You could share the experience, but I wouldn't really call that co-op, unless you're trying to learn something in a group setting about planetary ecosystems and balance.
It's the curiousity of "what ifs" when you mess with the planet. What if I added land life early? What if I tilted the whole planet? These questions keep you going and keep you wondering if there is something else you can do lead your planet to your version of success...or failure.
The fun is in the poking and proding of the different systems to see how things react. Will forcing evolution make a difference, or will it do nothing? What happens when you literally tilt the planet to be on it's side? These are the sorts of things that make the game fun.
The Less Fun
The waiting feels like waiting. In simulations, there is always an element of waiting, but there is also always something do to or watch while you wait. In SimCity that means you're building or planning new neighbourhoods, in SimAnt you're scouting out new food sources or figuring out where the red ants are in the map. In SimEarth, you're just watching data visualizations of the sim unfold and looking at charts and it feels like you're waiting.