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.
Even Better Sequel: While the original Alien is a great movie - interesting characters, creepy and horrifying designs for the alien, it introduced the xenomorph life cycle to an unsuspecting populace, and so on - the second movie is widely (though not universally) regarded as a better film. It also benefited from a Genre Shift from straight up Horror to Action Horror, which meant that instead of suffering from Sequelitis, Aliens was able to do things its own way.
Genre Turning Point: In American futuristic SF, the role of women was changed forever because of this film. Afterward, there was no room for any Neutral Female or Distressed Damsel in the future for any major female character; now they are expected to grab a weapon and join the fighting as much as any man.
They were overconfident and failed to set up appropriate backup plans before being stranded on LV-426. Justified in that Burke was pulling strings to place an inexperienced lieutenant in command that he could boss around and wanted someone to get infected so he could sneak Alien embryos back to Earth.
Why a massive spaceship is sent out with two dropships and for some reason nobody stays behind (is there no Navy in charge of running the ship?), not even a second platoon in case of emergency, is never explained. Maybe that's Burke manipulations again?
It Was His Sled: Can you believe that the Alien Queen was supposed to be a surprise? Nowadays it's so well-known that even people who barely know a thing about the Alien series know about it.
The phrase, "Nuke the site from orbit. It's the only way to be sure," is often used. A minor alteration is to simply tag "It's the only way to be sure" to any drastic suggestion.
Imitating Hudson's line, "Game over, man! Game over!" Or just about anything Hudson says really.
Keeping something handy "for close encounters."
"They mostly come at night. Mostly." That one was even in an episode of South Park.
"Get away from her, you bitch!"
Bishop's knife trick predates this film by hundreds of years, but is likely only well-known today as a result of it.
Moral Event Horizon: Burke locking Ripley and Newt in a soundproof room with two facehuggers so he can smuggle the alien embryos back to Earth.
Most Annoying Sound: Fans who dislike Newt usually point to her extremely, almost inhumanly high-pitched squeal, which sounds more like a tailpipe whistle than a screaming child.
Most Wonderful Sound: Fans love the high pitched shriek the aliens make so much that there was outrage that it wasn't included in Aliens: Colonial Marines.
Nightmare Fuel: The maddening and humiliating ordeal of the webbed colonists.... Now imaging if you will:
Being grabbed by a clawed gargoyle made of the very night itself, dragged kicking and screaming as it presses you onto a wall and traps you strand by strand beneath mucous that dries harder than rock. The hideous and bloated horseshoe crab smothered on the face of the colonist one foot away falls off dead, revealing him to be your best friend. Then you both watch in stunned horror as the girl stuck on the roof above you begins to choke as a fist-sized bulge begins to repeatedly punch outwards on her chest, until it ruptures in a shower of lung and bone fragments to reveal a viciously hissing, blood covered thing. Your friend, now wise to the awful finality of his fate, begins to weep in futility for the non-existent rescue, and finally given up struggling against his chitinous coffin, sadly begins to talk with you about happier days past on Earth, bitterly regretting each lost-opportunity wasted-chance and what could have been if he had more time, until his grief sours into a cold, tired resignation to the inevitable. You wake up with a start by his screams of "Someone Kill Me! PLEASE KILL ME!" that's quickly choked by his soon-shredded lungs, as the same fanged phallus begins to punch itself bloodily into the world through his heart. Nearby, a leathery pod placed next to you opens with a wet squelch like an obscne flower, as your friend's final gurgles are replaced by the vicious screech of yet another newborn monster. Eight spindly fingers lethargically haul the same horseshoe-crab that condemned him out of the abject soup of mucous and membranous web, and your final scream of grief with what little hope left inside is smothered as it leaps dripping towards your face, wraps its tail around your neck, and you pathetically try to stop the fat worm it forces down your throat... Then you wake for the last time in your coffin of resin, and realize that a woman in soldier's armor has lifted your limp and exhausted head; you feel the bloated serpent where the cavity between your heart and lungs should be begining to uncurl itself. Remembering your friend's last words, you echo him in sad resignation "Please... Kill Me..."''
Sequelitis: The film avoided this by using a completely different genre. While Alien was more horror/suspense, Aliens was a pretty straightforward sci-fi action film with a few moments of suspense/horror. Everything after the first two movies tries to copy one of those two formulas.
Special Effects Failure: Towards the end of the movie, when "torn-in-half" Bishop stretches to stop Newt from being sucked out an airlock, the hole in the floor he's actually standing in and his lower body are clearly visible.
Spiritual Licensee: Halo takes a great deal of inspiration from Aliens, including the space marines, the flying dropships, kinetic weapons, battles with parasitic aliens, and Sergeant Johnson, who is basically just Apone with a different name.
Aliens itself is often referred to as a stealth adaptation of Robert A. Heinlein's Starship Troopers — and a far superior adaptation to the later officially-licensed film. And even though it was just one suit, Aliens even had more Powered Armor than the actual Starship Troopers film franchise (at least until the third, straight-to-DVD film).
Tear Jerker: In the extended edition, Ripley learns her daughter had grown up and died while she was still in hypersleep. What's worse is that she died two years before the events of this movie. Her relationship with Newt is all the more meaningful in this context.
Ripley: (tearing up) Amy... I promised her... I'd be home for her birthday... her 11th birthday.
After Ripley learns Burke made the whole mess at LV-426 because he wanted a xenomorph, she should know better than basically say "I'm telling on you!" After all, she almost got killed doing the same thing in the first film.
Dropship pilot and gunner leave the loading ramp open and unguarded during their stand-by. They may have wanted to save time and fuel by landing close by instead of returning to ship or staying in the air but open ramp puts this into Too Dumb to Live territory.