Why is Lord of the Flies considered to be really well written?
Because it is!! Take this into account:
First, it offers a realistic view of the nature of humans that most writers (especially at this time) wouldn't touch. Golding not only offers the sentiment that we're all savages at heart, but he does so with a pack of British boys (who are supposed to all that is pure, innocent, and civil).
Secondly, Golding is a master of literary devices. He effectively characterizes all of the major characters, uses atmosphere during times of conflict, and litters the novel with irony. This might arguably be the best novel available for teaching the three forms of irony.
Thirdly, Golding has created a novel that can be studied by multiple reading levels for multiple reasons. It can be read by lower high school/junior high classes for a sense of plot and scene development, or it can be used with a senior honors class for study in theme.
Lastly, it's just a fantastic story. If you want to ignore the fact that Golding used many techniques that make English teachers drool, it's still a really good tale. I haven't found many students that don't like LOTF just as a story.