While it's visually not far removed from its predecessor, FIFA 10 has some under the hood changes that really improve the game. The biggest change is the introduction of '360 degree control'. Your players no longer move in just 8 directions, but whatever direction you want. This makes control a great deal more subtle, and gives you greater flexibility and the ability for much more creative play.
Video demonstration of the big new feature in FIFA 10: 360-degree dibbling. All in all: a more enjoyable, realistic gameplay with finer movements and more accurate passing.