Great turn-based tactical gameplay, great soundtrack, and destroyable mech parts to inhibit their functions (e.g. blowing an arm off disables the weapons on that arm). Learnable offensive, defensive, and support battle skills with their own animations to keep things more interesting than the usual "I shoot that guy with basic attack" gameplay. Equip new parts to learn new skills.
The story is quite deep, and branches at the very beginning so a second playthrough can have you on the opposing side. Also features New Game Plus, allowing you to carry over your pilot's learned battle skills and weapon experience into a new story.
Also has a very original in-universe "Internet" in the game, where you can learn more about the story and game's setting, and even try to hack government intelligence to learn what their plans are, and get tangible benefits such as new battle simulator arenas and more.