Personally, I just think it's a bit of a numbing effect. We've been horrified before, and it's going to take something really incredible to beat it -- and if it doesn't beat it, it does nothing. So many people told me The Ring Two scared the hell out of them... I saw it and was mostly irritated with how many times there were "Samara says BOO!" scenes. >.>

And at the same time, I thought The Grudge was one of the scariest movies I've ever seen. *shrugs* I dunno.