I really like Infinity a great deal and while I think all aspects of it are very good it really is the activation system that makes it great and sets it apart from the other games, especially WH40K. Firstly, it is a skirmish game using some where between 5 and 15 models, on a table using twice as much terrain.
The active player will select a model and begin to determine the orders it will follow but at the same time the opponent may make small reactions with any model that can see the active model, thus both players are always busy and always involved in the game. For me, this is the best table top game I have ever played. Additionally, skills, equipment and abilities are varied and extensive so that the same rosters on the same table will still have a different feel.