Not only is the story very different, Aliens is a completeley different genre - haunted house v combat movie with loads of subtext.
Ripley discovering her daughter is dead, setting up her mother/daughter relationship with Newt.
The Alien lifecycle/Queen establishined, Aliens easily killed when not based on a ship, aliens working together, none of which was alluded to in the original.
The batttle of the two mothers, following on from the earlier setup of Ripley wanting to protect Newt/her daughter vs the Queen and her offspring.
Seriously, most movie fans regard Aliens as one of the best sequels ever, because it is so different in how it expands the mythology of the first.
Weyland Yutani as a 'character'
The turnaround with Bishop being a good guy.
Three survivors - the 'family unit' continuing Cameron's theme even further.
Weapons technology vs biomechanical weapons.
Open battle vs sneaking around corridors (Aliens actually plays on our expectations of this when the marines enter the colony).
Besides the slightly 'one little indian' whittling of the marines, the two films are utterley different in theme, execution and mythology.
Alien 3 would be a better fit.
