The point that comes the most readily to mind when thinking about Ender's Game is the way military strategy is conceived of and executed. Of all of the Military Science Fiction novels that I've read, there are very few that really capture one of the major elements of combat, the strategic and leadership components that make a battlefield commander effective against an enemy force.
Humanity, under attack from an insectoid race known as the Formics (Buggers), fight tooth and nail to survive: first against a probing force, exploring our solar system, and a second time when a queen is to be installed for a permanent settlement. Earth is razed at points, and the countries of the world band together to form a cohesive military force to confront the military threat. In a number of ways, this is reminiscent of the overall strategy that worked effectively to combat the Axis powers during the Second World War.
Ender's education is the major element of the book that bears examination. From the beginning of the story, he understands a key component of warfare: render your enemy completely unable to pose a threat against you and your interests. Early on, he strikes out at a bully who is attempting to hurt him, noting later on that he would utterly destroy anyone who tries to harm him. It's a theme that's passed along through several points while at Battle School.
The same techniques apply as he is assigned to an army and begins learning the ropes to command an army of students on his own. There are several subsets to this: understanding just how to lead and most effectively lead the soldiers under his command, and effectively using those soldiers to carry out the goal in front of him.
From Bonzo, Ender learns what not to do: personal issues and ego can be exceptional motivators for success, but in his first army, Salamander, it's just as much a detriment to his own unit's cohesion, as Ender himself proves, when he breaks orders at one point to win a game. From other, fellow soldiers, such as Bean, he learns how it's important to use people at their most effective: while some might not be effective as platoon leaders, tapping into the strengths of those soldiers, and allowing them some degree of mobility or latitude to undertake parts of the army's overall mission. This essentially only works when there is a good deal of confidence in the abilities of the people below the leader. As equally important as leadership is, it's an essential key to delegate to sub-leaders who understand the end objective. In Bonzo's case, any strategy undertaken as such would likely not be effective. Under Ender, it propels him to the front of the standings amongst the students. This strategy would also prove effective when Ender is unknowingly fighting against the Formics, all the way to the end.
Militarily, Ender does a couple of interesting things: ensuring that any enemy can't strike back, but also learning from enemy soldiers and innovating accordingly. He moves around his soldiers into a different layout, allows a greater amount of latitude amongst his toon leaders, and uses his soldiers in unorthodox methods, such as using human shields or extra tools. These are key elements that real militaries utilize to undertake their missions, orienting themselves against one another by training to overcome individual systems and strategies.
With Ender's education, this is a military science fiction novel that does dual work with military strategy and tactics, both addressed at the same time, something that often doesn't happen in a novel in the genre. Furthermore, it does so within the framework of addressing some of the deepest moral elements in the genre: how does one even prepare for the total destruction of a race, and what's the impact? The fact that these elements are not only addressed, but really reinforced throughout the plot, make Ender's Game out to be one of the strongest within the genre.