Close Combat set a new standard in Real Time Strategy games.
It was based on the long running Squad Leader series of table-top simulations, and not only did it model weapons and terrain - but also the psychology of each individual soldier.
This was revolutionary because it meant that those little guys would no longer necessarily obey your every mouse click. The little soldiers react to events around them and interpret your orders according to their fatigue and morale. To players like myself who whetted their Real Time Strategy (RTS) teeth on Command and Conquer and Dark Reign, this added great subtlety to the gaming experience, or should I say it required much more subtlety in command to get results.
On the plus side, and this is pure joy.... the little fellows make intelligent use of any available cover, using trees, undergrowth, walls, buildings, trenches... any available cover. They'll also go to ground when under fire... crawling if necessary to limit their exposure to fire. Now this was a definite bonus!