I'm pretty sure Amityville Horror is considered a classic of the horror genre for the same reasons that James Joyce's Ulysses is considered one of literature: every critic back in the day praised it, and now modern-day critics and audiences are afraid to appear out of touch by bucking the trend.

I will say this though: you hit the nail on the head, talking about the opening scene where the DeFeo family is murdered. That is probably what sticks out the most about the movie to first-time viewers. It's a genuinely creepy, menacing start to the film, but I have to agree with you that it then goes on to squander that good-will by presenting a fairly generic, by-the-numbers Satanic Panic film, bolstered by the "Based on True Events" moniker.

One of the few horror films that I could see being much-improved with a remake/reboot, but even that wasn't such a great picture either. :)