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.
Angst? What Angst?: In BFE, Sam barely seems to register the early-game death of his squadmates as well as the frequent deaths of all the various allies he's supposed to meet up with. He doesn't even give the perfunctory "shit that sucks" that most other FPS protagonists give out after such events.
It also retcons his previous backstory, where it was implied he was suicidal (it was his ship that more or less brought Mental down on the human race).
The final level in Second Encounter is the biggest series of War Sequences in the...er, series. Until BFE came along with its final level, that is.
Big Lipped Alligator Moment - The House Of Sam trailers for Second Encounter HD, starring a unstable televangelist spreading the good word about The Second Coming Of Sam. Krundle Blind! KRUNDLE BLIND! KRUNDLE BLIND!!
Kleers. They come in packs, soak up damage, require a point-blank double-barrel shotgun to kill (which opens you up to attack from the other kleer), move fast, leap fast, and deal a huge chunk of damage.
Carpet bomber planes at the final boss of II are difficult to hit and even more difficult to avoid. Chaingunner zombies are like arachnoids in the first game, but they're much quicker when it comes to firing at you and they do more damage.
Technopolip-possessed Apache Helicopters are also troublesome since they're immune to bullets and some of them are encountered before getting the devastator or a cannon which is annoying since they tend to move out of the way of your rockets all the time. That, combined with their powerful hitscan machine gun, makes them incredibly frustrating enemies on higher difficulties.
Adult Arachnoids in BFE are a huge problem at higher difficulties, since they do more damage than Arachnoids in the previous games and tend to move out of the way of your rockets rather than standing still like their classic counterparts.
Fiendian Reptiloid Demons in The Second Encounter are very potent enemies. They have a very high amount of health, so much that it takes four sniper rounds to down them on Normal (practically every other enemy goes down in one or two), and launch extremely fast and damaging homing fireballs that can't be destroyed by anything other than the lasergun, cannon, or rocket launcher. They also have a punch that kills you in three hits if you have maxed out health and armor, but chances are you'll never even see it because eighty percent of the time they are placed in far off places or on top of buildings to spam their fireballs at you. If that wasn't enough, they never flinch, unlike ninety percent of the enemies, and they can use fireballs to block your bullets, which is not obvious upon first fighting them.
Headless Kamikazes may go down in one or two hits, but they're utterly relentless, and if one of them manages to get close enough to you, there's no escape; They will carve huge chunks out of your health meter when they go.
Game Breaker: Serious Bomb in Serious Sam 2, which instantly kills everything else within a quarter mile. Especially in multiplayer with unlimited ammo. They're pretty spam-proof in The Second Encounter, though, even being immune to the "giveall" cheat.
The Next Encounter has the sniper rifle... Oh god, the sniper rifle. Arguably, its might even surpasses the one SBC Cannon has, making it a whole ton less reliable.
Gameplay Derailment: Using your co-op mate as a platform to get the secret above you both. It stopped working since BFE came out, though.
Kleer in all games: They're vaguely bearable when they come in one or two, where you can unload rapid-fire ammo on them from afar, but anything more than three becomes Hell on Earth as they soak up all your bullets and swarm you with fast slashes that will kill you in three hits.
In Serious Sam I, marsh-hoppers, kamikazes and kleer, latter of which are the most common enemies... ever. Also, the Arachnoids, mostly due to being the only enemies in the game with hitscan projectiles. Sirian werebulls are like kleers but worse, given that they have more health and tend to knock you around in massive hordes while dealing high damage to your health, though it's also generally easier to avoid them. Aludran Reptiloids stayed back and fired fast, homing green energy balls at you, usually from a high vantage point. They tended to do this when you were in the middle of an encounter with melee enemies.
Marsh-Hoppers suddenly become much more manageable in Second Encounter. Just whip out the chainsaw and watch them jump right into it. Though there's still the occasional wave where they come at you from multiple directions and are harder to manage.
In Serious Sam 2, flying enemies, most notably witches, floaters, and harpies. Fatso Fighter and Seagull Bomber planes to some extent too due to being hard to hit but rather easy to avoid.
In Serious Sam 3, the kleers, werebulls, arachnoids, and kamikazes make a return, and several new annoying enemies are added. Some of the worst are the cave demons, cat-sized predators who leap around and lunge at Sam in dark areas. On Serious and Mental difficulties, the cloned soldiers (weak, inferior, cannon fodder versions of human troops) become this as well. They only take a few shots to kill, but there's tons of them, they have hitscan weaponry, they hide behind corners, and can slowly cut down your health while the goddamned kleers rush you.
There's also the Witch-Brides, Superpowered Mooks with telekinesis and invulnerable, damage absorbing barriers. Their telekinetic attacks do little damage, but they constantly teleport around and are invulnerable unless they're attacking, meaning they take far longer to die than they have a right to. What makes them so annoying is their 'choke' attack; they lift Sam up into the air, stun him, and inflict minor damage; this basically mean every single enemy in the area will instantly start ganging up on you until the attack is over while you are completely unable to defend yourself. On any difficulty above normal, getting hit by this attack is basically instant death.
The kleer skeletons step into Demonic Spiders territory during the early game, when they deal a disproportionately large amount of damage compared to everything else. The takedown move would be a great way to get rid of them if they didn't hunt in packs, since said move immobilizes you, so you'd better get used to that sledgehammer's timing and spacing pretty quickly. Towards the end of the game, they come in big enough packs that a sledgehammer isn't going to cut it. You'll find yourself breaking out heavy weapons like the Rockets and Devastator just to kill them before they can get to grips.
The first two levels of "The First Encounter" play out like a standard early FPS. Then you get to the end of third level, and suddenly hundreds of enemies start spawning all at once and running towards you. That's when the real game kicks in.
BFE starts you out very slowly. You have to use a hammer at first, don't even get a pistol until some way into the first level, and only get the serious (heh) firepower and battles after quite a while.
Mondegreen: "WAAAAAAAAR!!!" often gets interpreted as "MOOOOOOOOOORE!", "GOOOOOOOOORE!", and "WHOOOOOOOOOOOORE!", among other things.
Getting the "MEGA MIDNIGHT SECRET" in the Citadel in TSE (which contains Serious Damage and Invulnerability power-ups, a Minigun, and max health and armor pickups) normally. The entrance is only open for a very brief window of time, and that window starts so quickly that the only way to reach it in time is to recklessly speedrun your way through most of the level. Fortunately, if you know where the secret is, you can just pelt the entrance with many, many rockets and it'll open anyway.
That One Level / Best Level Ever - "The Grand Cathedral" from Second Encounter, while very impressive, is also clogged with enemies. The "Great Pyramid" from First Encounter, slightly less so. This is actually the best part for some players, since it gives a nice "Holy fuck!" feeling the first few times. The hardest difficulty also requires the players to perform some sort of slot puzzle in order to enter the pyramid to kill Ugh-Zan.
The final war sequence in Grand Cathedral is actually more forgiving as far as the respawning power-ups goes than the last arena fight in The Great Pyramid. The latter only gives you 2 +25 health pickups and one green ammo backpack, while the former gives you 2 +50 health and 2 +50 armor pickups, along with a green ammo backpack and a Serious Damage and Serious Speed pickup. Plus partway through the fight, mother nature lends a hand with the fight by raining fire and brimstone.
The Guardian of Time in BFE is basically both of those levels taken Up to Eleven.
Uncanny Valley: BFE cutscenes have a lot of this. Thankfully the rest of the budget seems to have gone into the 87 different War Sequences.
Visual Effects of Awesome: While they may not seem like much now, the first two games were technically lauded upon release for being able to render large hordes of enemies moving and attacking onscreen all at once without slowdown or frame rate jitters. Speaking on a more modern note, BFE has gotten note for having some very detailed graphics, especially considering that it's a downloadable game.