Defensively, the game is quite similar to the previous version, but it feels like the refs give you a little more leeway on slide tackles. You'll still get carded for egregious tackles and a hard regular tackle will still stop play with a free kick, but I like being able to go a little harder at offensive players when they get down near the box.
If you want to really get down and dirty with your strategies, FIFA 13 has the goods. You can play with formations and team behaviors to your heart's content or substitute in players you think will play a position better. As far as the details go, like every FIFA soccer game, this game is incredibly deep.
FIFA 13 also thankfully uses Apple's iCloud for syncing up between devices. If you choose, you can have the game save Tournaments and Manager Mode to iCloud so that you can pick up where you left off when you switch to another device.
One problem I found was specific to the iPhone. Thought there are a few different gameplay cameras to choose from, none move in much closer to the action. While strategically this is an advantage (being able to see more of the field) it forces you to bring your iPhone closer to your face to see the action. While very playable on an iPhone, the game is much better suited for the iPad.
Overall, FIFA 13 is an improvement on the previous versions in several ways and I think players of the previous versions will appreciate the changes. While the difference in skill levels is still pretty steep (as has been the case for all the FIFA games), the new easy-to-use skill moves make up for it with new ways to move around charging defenders.
If you played previous versions of FIFA or just want to check out what a great soccer game plays like on the touch screen, FIFA 13 is the best in its class and is easily the best sports game overall on iOS.