Aliens is certainly a really good film. I would have liked to say "A1 has depth and A2 doesnt" but that's simply not true; it has depth, but A2 belongs to that category of Monster Movie that is generally very shallow and the trope can be summarized thus:
1. A clearly visible monster with lots of detail and plenty of screentime
2. Action with both the monster and the hero doing a lot of damage
3. Generally a big showdown where the stronger monster is killed by wits or a gimmick
(T1 incidentally fallls right in the middle of this trope)
Because younger viewers love the monster, this style of film is mostly pandering to the easiest to please audience. Lots of work goes into the monster and action and very little work goes in the story. Due to the demands of how screentime is managed, and due to the characters suited for these plots, the result tends to be superficial rather than complex.
A1 is based on suspense - but also on a multitude of other factors which had never before existed of such quality in a scifi film.
As a thriller/horror, once you know the story, subsequent viewings will have a lessened impact, thats just the nature of the genre.
The most admirable traits of A1 ... hold on because we're gonna be here a while.
The excellent introduction, of the setting, the nostromo, the crew.
The casting, which successfully manages to NOT give away that "this is a film with a monster in it". A1 sets itself up a "serious" film.
The amazing storyboard, the alien spaceship, just about every set is groundbreaking.
The acting, with Ian Holm/Ash the android stealing the spotlight during the attack on ripley.
The pacing ... i want to say it's near perfect, with a bit of a wobble when they decide to fire up the shuttle.
..and frankly the direction is better.
What A1 suffers from, is that the Alien and the egg and facehugger, are so profundly ingrained in our culture, that the Alien film has already been spoiled for you even if you've never seen it. A1 depends on that surprise, that shock factor, that once you already know the surprise, you might not think of the reveal as the prize for having watched the slow, suspense-building parts.
I suppose A1 has gone down the same road that Citizen Kane has.