With the release of the Battlefield: Evolution Advanced Rulebook, Mongoose Publishing has demonstrated how a few simple rules can add up to a very elegant system. In fact, there are three rules/concepts in particular that deserve mention.
Actions In Battlefield: Evolution each unit gets two actions each turn. This can be two moves, move and shoot, ready a heavy weapon and shoot, or whatever. This simple concept allows for incredible levels of tactical flexibility, and causes the players to really think about their actions instead of just moving and shooting every turn. Also, it allows for two other concepts to be seamlessly brought into the game. When units come under a certain level of fire, they lose actions, thus getting pinned down. Meanwhile, leader figures grant extra actions, allowing well-led troops to overcome pinning.
D6/D10 The game uses two different types of dice. Although this means a few more dice are necessary, it allows for the same shooting/damage system to be used for both infantry and vehicles. This decreases complexity without compromising believability.
Reactions The best part about Battlefield: Evolution is the reaction system. Basically, any time a unit completes an action within 10 inches of an enemy unit, that enemy unit gets a free action. A unit also gets a free action if it is fired upon. This rule gives the game a great back and forth feel and serves as a built in over-watch system. However, units may only react once in any enemy turn, so sometimes you really have to think about when to react.
None of these rules seem revolutionary when written on paper, but when put together and applied on the tabletop, they combine to form one of the best wargaming systems ever produced.