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: Space Invaders
Anticlimax Boss: The true final battle of Infinity Gene. And then the solitary final invader, which is arguably harder than even the boss before it.
The Field ship. It slams anything within the targeting circle for extensive damage, and at max power, the circle is wide enough to cover the entire vertical height of the screen. It makes swarms of enemies practically trivial; any enemy that's a One-Hit-Point Wonder and is unfortunate to spawn within the circle won't even have a chance to appear on the screen!
Variable isn't quite as "kill everything around you" as Field, but any enemies caught in the laser swords is in for a world of pain. Even better if you get all four swords on a single target.
Classic, which kills anything in a single hit. Even bosses.
Genius Bonus: The names for some of the stages in Infinity Gene.
The very first stage, "Common Descent" is like that of an old-school arcade space invaders game. These invaders are the common ancestor of the other enemies you will soon face, hence the other enemies share a common descent.
The final stage (after the Final Boss), "Back Mutation" has you fight a single lone invader in an old-school arcade format. A back mutation is a mutation that reverts a gene back to its original self.
"Struggle for Existence" has a Boss Rush of 8 or so minibosses before encountering the real boss itself.
Good Bad Bugs: The point value of the flying saucer in the original version appeared random, but was actually a function of how many times the player had fired. A savvy player could count his shots, wait until his shot count got up to 14, and then shoot the saucer, guaranteeing himself a whopping 300 points.
In the original game, when the Invaders are at the bottom row, their bullets generate under the player's turret, rendering him immune to that row's attacks. This technique is called Nagoya Shot, so named for allegedly being discovered in Nagoya. Space Invaders Extreme references this with the "Nagoya Attack" bonus for Invaders that move in the traditional side-by-side pattern; Invaders destroyed on the bottom row yield a 20x score multiplier. Also referenced in Space Invaders Infinity Gene, where there's a point reward for exploiting a similar (deliberate) feature where enemy shots don't hurt you for the first second they're on the screen, once you unlock it.
That One Boss: The final boss of Extreme 2 at at least 5-D is the closest Space Invaders can come to Bullet Hell. It can use a ton of different shots and has three seperate targets, each with its own massive amount of health. Extreme 5-C is considered to be one of the, if not the, hardest bosses in the franchise, and non-Bullet Hell shooters as a whole.