Poltergeist really got me when I first saw it as a teen, but hasn't had as much effect in subsequent viewings. The effects don't quite hold up, and a lot of the "scary" parts rely on unexpected shock, so once you know they're coming they don't hold as much power. That said, the last time I watched it (a few months ago), I was surprised at how much of it was still effective.

Blair Witch didn't get me for the most part (I saw it after months of following the hype, near the end of its theatrical life, and was doing it for a review, so I pretty much knew what I was getting), but man, that final sequence--that's pretty unsettling.

That entry I made on Halloween last year about what movies have scared me was an interesting exercise in how my perception of "scary" has shifted with age and experience. Gore films don't really get me anymore, because I can distance myself from the visceral aspects (pun kind of intended) and recognize that it's not real, but for that reason I'm not as interested in those. Suspense or psychological horror is much more likely to have a lasting effect on me now, but perversely that makes me prefer it, because there's a greater chance I'll be immersed in it rather than standing aside and critiquing the effects work. ;)