This is a very broad question, and clearly in any novel or play there are going to be plenty of events that are very important to the overall plot. However, for me, one vital element of this excellent story lies in the way that we discover, as Ender does at the end, that the "game" he has been playing has not, in fact, been a "game" at all, but has been a real life and death struggle against the buggers, and that Ender's winning of the game actually represents the end of the war against the buggers and also the annihilation of an entire species. This of course raises the level of abuse that the military carry out on Ender to a new level, as keeping him in ignorance meant that he committed a crime he never wanted to perpetrate. Consider what Ender says the day after his victory:
I didn't want to kill them all. I didn't want to kill anybody! I am not a killer! You didn't want me, you bastards, you wanted Peter, but you made me do it, you tricked me into it!
This fact takes the title, Ender's Game, to an entirely new level of meaning, as not only was Ender under the impression that he was playing just a game, but also he was being played with by the military.