These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Immortal Defense
Crowning Music of Awesome - The soundtrack varies between eerie, ethereal and heartpumping tracks randomnly chosen for the main stages, and kicks it up a notch for the act finales. The last standard level of the game has an entirely unique, twelve minute-long piece added in a patch. And then the final secret stage uses the achingly sad title screen music to full effect.
Those accursed ":P" enemies. They are completely invincible as their default state, and only become vulnerable for brief points while their shields recharge. Thankfully, they don't usually have much health, but they can easily slip through your defenses if you don't keep an eye on them.
Goddamned Bats - In the final campaign area, you get accosted by these little flying... things that look like the love child of a Fairy Princess, Cthulhu, and one of those fancy plasma globes. They can't be destroyed or even targeted, and just love to come in and suck the power right out of your points...
Tear Jerker - Your betrayal of Corybantes in order to please the voices in your head. "God, Great Star...I loved you so much!"
Also, discovering in the final campaign that Corybantes' attempts to form a peaceful republic were for naught, as they've become an oppressive empire once again.
That One Boss - Most people seem to have trouble fighting the Panopticon. Understandable, since it's Nigh Invulnerable to begin with and quickly forms an extremely potent shield that blocks all incoming bullets.
The final, secret campaign contains a lot of Buddhist imagery and references, specifically to the Koan of Gutei's Finger. However, given the direction of the story up to this point, it's debatable whether or not these references are truly arbitrary or superfluous; the original creator certainly didn't consider them such.