Given that they really are just monsters, none of the playable characters have much in terms of personality, leaving a lot to interpretation.
Is Goliath Prime specifically built to destroy you, or is it just another out of control metal beast?
Were the invaders really a massive armada, or was it just Cerebulon and the three Zorgulons with a lot of remote control ships?
Is Cerebulon the leader of the aliens, or simply their ultimate weapon?
Awesome Art: The In-Universe film posters were obviously made with as much love for giant monster films as the rest of the game.
Breather Boss: Between the frustrating battles with Goliath Prime and Cerebulon we have Vegon, who may be the largest monster in the whole game, but isn't really much trouble so long as the player keeps moving.
Crowning Moment of Awesome: For whatever monster you completed adventure mode with. After taking on (presumably) every other monster in the world, you fight the alien leader, and beat the daylights outta him too!
Crowning Music of Awesome: While you often can't hear it due to the explosions and buildings collapsing, it is there. Most of it plays through the credits.
Capitol: Awesome music for a last stand against a stronger enemy.
Demonic Spiders: Military vehicles and flying saucers in story mode. Both are tedious to destroy, and will chip away at your health while you try to fight your opponent.
Fanfic Fuel: The plot is extremely minimal, leaving a lot of gaps to be filled in by fans.
Beating the game with Raptros or Zorgulon unlocks a Making Of Video, rather than an Origin Movie. Some fans have had great fun speculating their origins.
Goddamned Bats: The constantly-respawning Kineticlopses in Atomic Island, as well as the military's helicopters.
That One Boss: Goliath Prime, the first boss. The fact that he's big enough that other Kaiju consider him a boss should tell you something. The first third of the battle is a mild Guide Dang It moment (you have to catch the bombs he throws at you), and the rest of the fight alternates between him pummeling you, trampling you, or blasting you with cannons.
Also Cerebulon, the Final Boss. In his first form he fires a beam cannon that drains half your health, and about the only way you can avoid taking damage from it is to hide behind and object and let it take some of the hit. In his second form he invokes Instant Death Radius with a red lightning attack that drains half your health (to say nothing of how hard it is to actually damage this form). In his last form he uses a 12-hit combo that stunlocks you and, you guessed it, drains half your health.
That One Level: The Airfield. To summarize: a two-against-one battle. Small area with virtually no cover, but plenty of explosive objects to be thrown at you. Health pickups aren't more than 5 seconds away, no matter where you are, which means the enemy will eagerly grab them the second they start taking damage. And they think nothing of throwing explosives and spearing weapons non-stop.
Metro City has the opposite problem during story mode. The level is home to Preytor, who will fly off to the other side of the map, or to the top of the tallest building if you don't defeat them as soon as possible. The fact that you also have to contend with Mecha-Congar doesn't help.