Amazing Technicolor Battlefield - The entire game, particularly the later levels. Especially noticeable at the end of levels, when everything starts to distort and leave rainbow-colored trails everywhere.
Big Damn Heroes - Of all the places to find this trope...in the final level of the normal game, you have access to all your points save one. And just when you are about to be overwhelmed by impossible odds, the Strategist point cries, 'The sleeping dragon...is here!' and you can place it to get the upper hand again.
Gainax Ending / Downer Ending - You only get a single line: "I'll always love you, Grandpa...forever..." Then cue the credits. Have you gone insane, or what? The Raberata who runs the training ground puts it best: "I'm not sure whether you accomplished anything good or bad by defending your dead homeworld, but at least you did it well." (This also serves as your single hint on how to unlock the Bonus Level Of Hell.)
The secret ending is even worse. You only get a single picture: Gutei holding up his finger.
Heroic Mime - Subject K, yourself. You never say a thing throughout the entire game. Though your Points are pretty chatty, and they are aspects of you. Plus, the hallucinations after you go mad.
Hold the Line - This is pretty much the entire point of a path defender's existence.
This is also how gameplay works, instead of simply destroying everything, a more typical variant of Tower Defense games. You only have to hold off the opposition until the timer runs out.
Hopeless Boss Fight - The second Bavakh Brother, who destroys your home planet. Even if you somehow manage to defeat him, the story proceeds as though you lost.
Ducrosh's mission was to deliver the bomb to Dukis, and he's already there by the time the mission begins. It doesn't matter whether he's subsequently killed by K or the explosion.
Hyper Space Is A Scary Place - Pathspace is a pretty bleak place to live. It only gets worse once reality starts breaking down around you. Or maybe it's not, and you're just imagining it all? And from the perspective of your enemies, Hyperspace itself is the place where immortal, invulnerable ghosts endlessly tear apart your fleets, first in the defense of their homes, but later because they've gone batshit insane.
Not So Different - As your character's behavior becomes more and more erratic, multiple explicit comparisons are made between him and Aa.
Power Born of Madness - Represented by the Turning Points. They're erratic and unpredictable, making them impossible to use reliably, but can still have a number of useful abilities. They're also the only points who can actually move after being placed.
Power of Friendship - Represented by the Circuit Points. These points used to be the weakest and least useful, but were recently Rescued from the Scrappy Heap ... perhaps a little too well, as they're pretty much overpowered as of the V1.1 patch, able to punch through everything except enemies on the extreme high end of the power scale, and those accursed Triad Flash enemies.
Power of Love - The Love points are your standard "support" towers, increasing the range and attack power of nearby points (and you, if you're next to one). Unless you upgrade them fully, in which case they double the attack and range of all your towers all over the field.
This is played somewhat straighter in the "secret" ending of the game, where it's nothing but you, Aa, an inexplicable old lady, Gutei's finger, and a tug of war between your love points and Aa's hate points...
Power Nullifier - While Bavakh can clear out all bullets in pathspace, there is a regular ship appearing later that disables a close point.
Through the Eyes of Madness - As the game progresses, you are led to question whether your character is truly defending what he thinks he's defending, or whether he's simply gone crazy. Then, once it's become apparent he's gone crazy, the game starts raising the question of when he went crazy.
Trailers Always Spoil - The trailer spoils the fact that path defenders really are ghosts and can't ever return to their bodies. It also shows a shot of the final level, though that isn't terribly spoiler-y without context.
Unwinnable - The absolute final level, Level CI. No enemies, no points, no cursor, no HP, just an empty path and an immediate Mission Failed screen when you start the level. Originally a bug, it was left in because the devs thought it expressed the story's Aesop nicely.
Video Game Set Piece - The final level of the "normal" game. The path moves. Well, though the individual path doesn't change shape, the whole thing rotates. This also prevents you from placing points anywhere except the perimeter. The timer is also far longer than any prior level.
Western Zodiac - Some of the Points are represented by Zodiac symbols.
Fear Points: Gemini
Ortho Points: Tauros
Cut Points: Pisces
Wham Episode - The end of the second campaign. Aa sells out your planet to the Bavakh, and it — along with your physical body — is completely destroyed.
And there's an even bigger one at the end of the fifth campaign. After a long period of weirdness and the game questioning your sanity, the ending finally confirms it: you've gone completely insane, and your "granddaughter's" messages are entirely in your head.
Who Wants to Live Forever? - The entire game is essentially one long exploration of what the path defenders do with their immortality.
On the other hand, your granddaughter is happy to live for millennia after the Dukis scientists discover the technology which allows that. Of course, it turns out she never existed.
With Great Power Comes Great Insanity - Sure you're single-handedly more than a match for any fleet in existence, as well as ancient all-destroying and unknowable gods. However, it's heavily implied (if not outright stated) that eventually all path defenders go insane, and spend the rest of their immortal existences obsessing over something. Aa gets delusions of godhood and demands worship from his people, the inventor of pathspace technology tinkers endlessly with his shooting galleries, and of course your character spends hundreds of thousands of years defending the dead rock that used to be his home planet.
Yandere - You, to your "granddaughter" and dead homeworld, mercilessly destroying any ships that come near where Dukis used to be, even when they're completely harmless.