Demonic Spiders: Literally! Giant spiders can leap through the air like grasshoppers and fire life-sapping strands of silk that pulls you towards them, in addition to leaving webs around that can also drain your health if you get caught in them, thus hampering your movement.
Most Wonderful Sound: The six "EDF" chants in the voice command list. If you're leading an AI squad, using it causes all of them to instantly chant "EDF" with you, as if on their own accord. Chants of EDF! EDF! will override all other voice lines and drown out all other sound.
To a lesser extent, if you have a squad, you can issue commands to them with the multiplayer voice commands. Granted, they won't actually obey them, but hearing them respond "Yes sir!" in unison to you at least makes you feel in command.
Narm Charm: Earth Defense Force 2025 is a giant love letter to the science fiction B-movies of the past. Naturally this extends to the dialogue; every line of dialogue is either purposefully overdramatic or completely failing to be dramatic. Most of this is due to EDF troops making macho boasts without the macho voice to match, making them come off like complete dorks.
Nightmare Fuel: The third trailer of the game introduces an enemy type that resembles the Asian giant hornet, the world's largest and dangerous hornets known to man.
That One Level: Mission 23, "Giant Robots", is the first mission that will likely take you several tries. You have to fight off three waves of Hectors, many of which are the deadly plasma cannon and rocket variants that can knock you around like a football as well as severely damage you, and even the ones who "just" have machine guns can chew through your health like paper. In addition, the Hectors will more often than not reduce your NPC squadmates to mulch in a matter of seconds, meaning you'll most likely end up having to fight the Hectors and their devastating weapons all by yourself.
Polished Port: The Steam release of 4.1 performs even better than the PS4 version with an actual stable 60 FPS like what was promised in the PS4 version even in high-intensity scenes on mid-range hardware, as long as you're using an Intel CPU. As of July 2016, it is unusable with AMD CPUs.