I've gathered (a mate's a HUGE Aliens fan) that most of what was considered a letdown about the third movie was how it rendered all of the comics, all the supporting material, everything, no longer canon or canon-compliant at one stroke. It effectively squatted a huge amount of fandom-acclaimed and well-written material. The only other movie as hated by a fandom I can think of is Mad Max Beyond Thunderdome, which had exactly nothing to do with Mad Max.
(Incidentally, am fully expecting Fury Road to be even worse than Blunderdome. My bro reports an absence of Aussie accents in the trailer - FAIL.)