Aliens visually and content wise closely emulates Alien (and Them! and Starship Troopers) which is no biggy as Alien was in terms of plot practically steals from many a 50's B Movie, only with such Panache it really doesn't matter.
The action movie tone is the key difference (and the crappy aspect ratio and back-projection).
Cameron pinched all the lighting effects and the basic plot structure (replacing the one perfect organism with one bigger perfect organism with lots of squishable ones).
Aliens is visually more clumsy though and some of the performances are too cheesy.
The film I would love to see is Vincent Ward's Alien 3 because that would have really have been a visual departure from the first film and a completely unique story (but I'd have to jump through the dimension gate to find it).