Lost was an unusual show because a lot of the hardcore fans would discuss the plot and mystery between episodes. It was pretty hardcore back then, lots of screenshotting and working shit out on forums. They also added a reality game during seasons which people played on the internet. When you provide fans with that level of complexity and they threw out a half assed ending, then expect the fans to be insulted by it.
I remember in the early seasons in particular the producers and writers would frequently release interviews stating that the show would make sense and the common theory behind the show wasn't correct. They lied. Whoops.
I do appreciate the character arcs in the show though and the initial mystery was amazing. When Lost was good, it was exceptional. Just faded near the end as the writers struggled to make sense of it all. They fell into the trap of dragging the show out for too long because it was so successful.