People have such a visceral reaction to this show because majority of it was good. The developed interesting characters and threw them into a survival situation with a slight paranormal (answered by weird science) setting. For the most part it was grounded in reality.
The backlash because the writers lied to the viewers all along about significant parts of what was going to happen. In interviews, live panels, documentary pieces, they told fans- oh yes, x, y, and z be answered.
All the mysteries in seasons 1-4, which could have simple been kept to DHARMA and the Others tapping into an unexplained power, with the goal of getting away from these crazies instead was bait and switched to be a religious purgatory allegory. That Jacob, a god like figure hand been torturing the main character and thousands people before them because he was looking for the right one to succeed him. Which ends up mentally destroying the man of science, Jack, into a crazy, sociopath murder. Which is even more "what" considering Jack was suppose to die in the pilot.
Using faith and religious allegory, they tried to cover up the fact that they for whatever reason wrote themselves into a corner with the Others and Dharma. The put numerous Checkoff's guns into the early seasons that they completely dropped in favor of a completely different premise. Which is absurd because their was so much fertile material their they could of crafted and ending with- it boggles the mind why they switched gears and eviscerated the story they had been telling for 4 seasons.
The end retroactive made the characters journey hollow. Nothing they did matter, cause in the end- none of them had free will. None of their choices mattered, because Jacob controlled them like pawns. The story ended up being Jacob and Smokey being similar to God and Lucifer playing with the life of Job.
The writers in the end not only tossed the mysteries into the bin, they made it that there is no point going back an watching the series as a whole ever again because the characters never will get a satisfying resolution.