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.
Breather Level - Subverted in Salamander. Stage 3 has a soundtrack to match the terrifying and chaotic nature of the level, but it's otherwise not too bad of a stage. Stage 4 has a calm, soothing soundtrack not unlike going down a river in an inner tube, suggesting the game is setting this trope up. It isn't..
Crowning Music of Awesome - Gradius contains a large amount, listing them here would take too much space. Examples can be seen on the trope page.
Demonic Spiders - Those bouncy things that shoot omnidirectional bullet spams, the Bubbles in the titular stage, especially in IV (where you have bouncing icebergs to dodge as well), the dreaded Option Thief, the goddamned Zubs (who sometimes materialize on top of you), the inflating mini-Moais that the Moai Boss spawns, the fireballs that generate indestructible shrapnel, and the dreaded Cube Rush at the end of the Ice Stage.
Ear Worm - Aircraft Carrier aka the boss theme in the first Gradius aka the first theme of the boss rushes. It is literally only 4 measures that loop over and over again but god damn it if it isn't catchy.
Game Breaker - Some of the custom weapon set-ups can be this if the right weapons and option formations are used.
The Falchion Beta is this in Gaiden. The Auto-Aiming at full power covers 180 degrees of the top of your ship, the Gravity Bullet speed kills bosses, and the Rolling Missile both goes in both directions and pierces through weak enemies.
Goddamned Bats - Lots of examples. One particularly irritating example the "Strobe Ship" in Salamander's second and fifth stages, which Inferface Screws you with seizure-inducing flashes.
It's Hard, so It Sucks: The series was already infernally difficult to begin with but Gradius III is just so preposterously difficult that even hardcore Gradius fans started to turn away from it. Even one of the developers made the Konami Code for that game because he admitted that he couldn't beat it in one run.
Well, you do have to beat the game three times in order to actually beat it completely with an ending.
Magnificent Bastard: Paula in the Salamander OVAs. She poses as a boat person from the 5th planet and tricks one of the protagonists into destroying a Moai Statue, which allows the Bacterians, who were repelled by the Moai Statue, to come in and mess up the place.
Moral Event Horizon: In Nemesis 3, Venom attacked planet Gradius and kidnapped a three-year old James Burton to change history in favor of the Bacterians. His attack has indirectly killed many of Venom's own species due to radiation.
"You think you can beat me with THAT? Loser!" —Moai Dimension, Gradius Gaiden stage 4
"I'm going to scare you like you've never been scared before!" —Doom, Salamander 2 stage 6
Nightmare Fuel - Usually the Bacterian Emperor either looks like a Brain, a Giant Eye (Zelos), a skinless head (Gofer), a three headed humanoid (Doom), or a face surrounded by smaller faces (Bacterion).
There is also the two facts that :
A. The Bacterians are pieces of Bacterion who is an cell.
B. The Homeworld of the Bacterians is Bacterion himself.
What's really scary is that every the Bacterian Emperor gets defeated, the cells composing the Bacterian Empire increase in numbers, making a new and stronger Empire.
Polished Port: Gradius II on PC Engine is a very faithful recreation of the arcade version, AND adds a new stage. The Famicom verson, while not one-to-one like the PCE counterpart, stands out on its own with its unique stage design, and is the only Gradius game on the Famicom/NES to allow four options on the screen at oncenote To compare, Gradius and Life Force (NA/PAL) only allow two, and Salamander allows three. thanks to the cartridge's special chip.
Scrappy Level The Fire and Ice zones in III Arcade, the Bubble and Cell stages in IV, and the "green slime factory" stage in V. And the fortress stage in almost every game. ALL of Gradius III arcade.
The first stage of III Arcade is pretty fun, and provided you played perfectly (you have four options, your laser, and missiles) and keep up a steady stream of fire, the second stage is tense, but enjoyable. It's the third stage where it all goes downhill (ironically, the third stage is virtually a rehash of the first stage of I only a hell of a lot harder, and appears again in Gradius Rebirth, where it is nowhere near as hard).
Gradius III is not only atrociously difficult, but it's also full of extremely cheap moments that just make the game not very fun at all. The difficulty had to be toned down very drastically for the SNES port. Many players regard it as the worst game in the mainline Gradius series.
Gradius IV is victim to this, due to not adding anything new to the series (the first stage is even a re-hash of the first stage from Gradius II) and for the 3D effects looking rather cheap.
So Cool It's Awesome: The entire series, but fans consider Gradius Gaiden to be the best of the series, or at least one of the best. Also, Treasure was involved in the making of Gradius V, and the game has a little resemblance to Ikaruga.
That One Boss - Big Core MK III and the Moai boss in Gradius III Arcade/[PS2], Heaven's Gate in Gaiden(possibly the hardest in the series), Belial(a suped-up remake of Golem from Salamander/Life Force) in Gradius IV, Bolboros, the twin ships from Gradius Galaxies/Generations/Advance and most of the bosses in V, particularly the remake of Beacon from Gradius III SNES.
How can we not have mentioned the Boss from Gradius 4's High speed stage...already a frustratingly difficult stage, they have the nerve to throw a boss this troper and his friends have dubbed "Suckbot" a half eaten donut with small laser guns filling all the empty space down the flat side facing you, the "donut hole" core aims independantly of the outer shell, so it fires around 10 lasers in one direction and another 4-6 in another... and every time you think you've got his pattern figured out...he throws something new at you, like turning the core sideways and blastoing you with a no "other" warning laser blast. And then Konami has the gall to bring "Suckbot" back for a rematch in a BOSS RUSH in Gradius 5.