This!
First and foremost, Spielberg didn't insult his audience or try to show them how clever he was. And no matter how many war movies you'd seen prior to SPR, NOTHING (short of being a veteran of actual combat) could possibly prepare you for that first 30 minutes. A cinematic masterpiece is one that affects you like no other film has ever affected you before. Spielberg took a well worn story about an event that many movies before have tried to recreate, and did something totally different with it. It blew the audience away and every scene that comes after that opening sequence is built on that visceral reaction. I've seen lots of war movies, read lots of books about war, and have a high tolerance for blood and guts but the first time I saw that in a theatre, when I saw that soldier on the beach who had been disemboweled, holding his bloody entrails in his hands and screaming for his mother, I was genuinely disturbed. That's what a great movie can do. That's the power it has in the hands of a skillful director.
Thin Red Line just bored the shit out of me. The only power it had was to induce the desire in me to take a nap.