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.
Better on DVD: The extended cut restores Newt's backstory, where her father was the first colonist who was infected, and Ripley's subplot of her heartbreak over losing her daughter during hypersleep, thereby adding a much deeper dimension to her relationship with Newt. These additions do interfere with the film's pacing, however; it's up to the individual viewer's opinion as to whether or not the trade-off is worth it.
Broken Base: This movie is either considered to be a worthy followup to the original or the inferior to the original, but critically it is considered better than anything after it.
This is especially prominent on IMDB, where the user reviews range from the majority of very positive to the Vocal Minority of outright hate.
Discredited Meme: Carrie Henn, who played Newt, has said that she hates the line "They mostly come at night. Mostly."
Bishop, to the point that he's basically the second main character of the Alien franchise after Ripley.
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 Damsel in Distress 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?
The Scrappy: Some find Newt to be one. Some fans even supported Alien³ killing her off partly because of this.
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 Effect 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. This was fixed in the Blu-Ray release of the film.
More of an editing failure, but prior to the Blu-ray release there's a scene where Ripley takes a Pulse Rifle and Flamethrower off the rack and tapes them together over 4 shots. The failure is in the weapons swapping when she places them down. (She grabs a flamethrower and puts down a rifle, then vice versa).
The dropship suffers from some very obvious green screen matting at some points.
When Ripley and Newt are trapped with the facehuggers and Newt tells Ripley to break the glass of the window, the "glass" rebounds from the chair just like plastic.
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).
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.
Ripley and Burke for not speaking up to explain to the Marines why they had to switch from pulse rifles to flame throwers. Well, that might have been Burke following his own agenda, but it's less justified with Ripley.