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.
Lou Guards (green hulks armed with homing missiles), Diamond Claws (sharp claws of course, sneak up on you, often double team you, sometimes cloaked, and they shoot plasma back when hit with energy weapons), Sidearm Modulas (tiny and hard to hit, fire Macross Flash Missile Massacres and are produced when Sidearms are destroyed), Seekers (tough as nails, fire fast Mercury Missiles that take off a large chunk your shields, tend to camp around corners, major pain on Quartzon), the Energy Bandit, which does Exactly What It Says on the Tin, draining both your shields and energy, and the dreaded Boarshead, the deadliest normal enemy in D2, which is armed with a gatling plasma cannon and Smart Missiles.
Let's not forget the Stingers, which are basically Descent 3's even more dangerous replacement for Lou Guards. Stingers are hard to hit, extremely aggressive, and they can spam a ridiculous number of homing missiles-and, to top it off, they give off a horrible screeching sound when they find you. The level 3 version of those Stingers are a pain to beat since they're a lot faster (insanely faster in higher difficulties), shoots 2 duel homing missiles at you, and their claws inflicts more damage.
The BPER Bot's phase pulse cannon (same as the Sidearm's, but faster) drains your shields very quickly, and it also can kill from beyond the grave.
Omega Defense Spawn. Small, fast, tougher than they look, and they explode on top of you as their attack. Even when detonated well away from you, the shockwave is enormous, and tends to throw your ship violently.
And good old Descent had Fusion Hulks, each one a Boss in Mook Clothing (an alternate name for them is "Mini-boss"), Red Hulks, which the aforementioned Lou Guards are identicalExpys of (although Red Hulks seem more durable because Descent 1 doesn't have as many powerful weapons to finish then quickly), and Class 1 Drillers, who had the nigh-undodgeable Vulcan Cannon as their armament. To make matters worse, the Drillers later came in a slightly more durable cloaked variety.
The tail bots are quite annoying. Level 1 tail bots are a joke (unless playing higher difficulty) that fires EMP lasers at you, while level 2 fires concussion missiles and level 3 fires Frag missiles that can kill you instantly if you're near a wall.
Heavy Drillers, which debut in large numbers in Level 12, creating a major Difficulty Spike up from the already hard Level 11. Vertigo Series has Class 2 Heavy Drillers which have returning homing plasma.
The Advanced Lifter in the first game was lightning fast, did considerable damage with its four claws, and was the only bot that never made a sound. Get used to turning around and seeing a pack of them right on your tail.
Let's just say that ANY robot in the first two Descent games will, in some way or another, play this trope or Goddamn Bats. Even the frickin' Guide Bot. Descent 3....more or less so.......
The Helix Cannon is a massive upgrade to the Spreadfire with greatly increased damage, rate of fire, and spread size. Though it consumes a lot of energy, it can mow down nearly any robot in seconds. In levels with a large amount of energy centers, the Helix is ridiculously powerful.
And then there's the Smart Mines, which take the merely-annoying Proximity Bombs and add in a crapton of homing projectiles, enough that one bomb can one-shot almost any Mook provided all the projectiles home in on the same target.
Gameplay Derailment: A small oversight in Secret Level 4 of Descent II completely changes the way the level plays. Normally, blowing up the reactor at the beginning gives you an extended countdown (125 to 375 seconds) to search for as many powerups as possible in the massive level, but since the initial grated door is only locked from your side, it can be opened from behind with a Guided Missile to skip blowing up the reactor completely.
The Thief Bot in Descent II, one in each level. This nasty bot would come sneaking up on an unsuspecting player — typically in the middle of a fight with other enemies. It would zap you with a special shot that made your view go all wonky, steal some powerups, and then zip off to the furthest corners of the level in an erratic evasion pattern. Incredibly fast and durable.
Internal Tactical Droids (which are Demonic Spiders on Insane difficulty), Sidearm Modulas (pesky little things that shoot flash missiles), ITSCs (like ITDs, but with missiles), Red Hornets, etc.
The Thief changes in Descent 3, in that it can cloak. As an upside, it can no longer screw your interface, but now comes with a weapon in the form of Seeker Mines.
Slightly less annoying is the "Old Scratch" robot in Descent 3, which tears off your weapons, but simply leaves them floating nearby. It's also much easier to kill.
Remember those bosses which teleport around the arena, often shoot homing projectiles and make an eerie ticking engine sound?
In Descent 3, the "Old Scratch" type robots have a dark gray paintjob that blends in extremely well with the game's frequently gloomy environments, and they're quite stealthy aside from a quiet, creepy noise they make occasionally. Hearing that noise in a dark cave tends to result in player panic and frantically looking around for said robot... which has a nasty tendency to then pop up right next to the player and scare the living daylights out of them.
Descent 2's Diamond Claw did this, too, except they didn't make that idle clicking noise. Just silence between the periodic hrumphmm...hrumphmm as it slowly approached the player. By the time you figure out where he i- CLANG! OHMYGOD!!!
In fact, almost every enemy robot (especially the old scratches, level 3 stingers, etc) can spell Nightmare Fuel for almost anyone playing Descent 3.
Oh, you might say, but most of these examples deal with robots that sneak up on you. Maybe if I turn off the music, I can hear them coming and they'll be less scary, right? Wrong. Those robots are still just as hard to locate, even without the soundtrack. You still don't know where they are or where they're coming from. Only now, you have a backdrop of near-total silence to enhance the fear factor. Hrumphmm...
That One Boss - Many of the bosses, due to being armed with the most powerful weapons in the game on top of being accompanied by a small army of Mooks, and some of whom are Mook Makers to boot.
In Descent, the Super Hulk was tough enough, until you learned that its shots don't track well when circling around the center pillar, and that it's possible to move freely in all directions without turning. Regardless, both bosses in that game tend to land a One-Hit Kill without a direct hit.
The available powerups made the Super Hulk in Level 7 a lot harder than the End Hulk in the last level, though. By the time you face the Super Hulk, your strongest weapons are the Spreadfire Cannon and Homing Missiles, but you get Smart Missiles and Mega Missiles (which the respective bosses use) within the next few levels. You also have a lot more available invulnerability and cloaking powerups in the final battle, instead of one each and a few shield boosts in the Super Hulk battle... and did we mention you're locked in the boss room upon entry in Level 7, but in the final level, you're not?
The Ice Boss from Descent II takes the cake for pure frustration. He fires homing Flash missles (which blind you) and an Omega Cannon (the strongest primary weapon in the game) which has been haxxored so that it also blinds you. Meaning that unless you stay up in the passageway leading into the Boss Room (which protects you from most of its attacks), you'll likely be flying completely blind for the entire battle. It's also immune to energy weapons, meaning that the only two primary weapons that even affect him are the Vulcan and Gauss cannons, for which ammo is rather limited.
The Homonculus in the third game: it's ridiculously fast for its size, it can rip the player apart in seconds if it gets close, it can spam powerful homing projectiles from afar, it's accompanied by a whole damn swarm of other robots (including the aforementioned Old Scratch type), and there's no place to hide in its chamber. Oh, and it's just a little over the third of the game in, when the player doesn't even have any powerful weapons.
That One Level - Level 6, Level 11, Level 19 and Level 26 tend to be this in the first game. All are chock full of Demonic Spiders and none have all that much hiding spaceó for example, Level 19's circular design means you could be attacked from any direction, and in Level 26, you're forced to retrace a narrow set of tunnels whose crossroads trigger the only Fusion Hulk Mook Maker in the game... and they can shoot at you from outside the tunnel. There are token cloaking devices or invulnerability globes, but predictably in hard-to-reach places.
They Changed It, Now It Sucks - Descent 3's addition of gravity and change from flight-sim style aiming to FPS style aiming prompted revolts from fans of the first 2 games. Some gamers also cried this because most of the weapons in Descent 3 were either useless or much weaker than the ones in Descent and Descent II.
Players who preferred to play with joysticks didn't like that players who used their mouse and keyboard to control (read: almost everyone who played a First-Person Shooter) were a lot better at aiming and rapidly switching targets. The default configuration for a Descent 3 multiplayer server did not allow people using mouse and keyboard to play.
Descent 3 defaults the turning system to "Flight Sim", not "Mouselook", so there's some latency to the turning.
What about the increased turn speeds that Spacetec Space Orb 360 users had access to in the first two games?