A LOT.
It's not image quality I'm talking about, rather, appearance of darkness. I remember when I got my first LCD (on a gaming laptop, actually). I was crushed by how terrible the black levels were. Unfortunately, this has remained an issue for years.
Silent Hill, for instance, was scary as hell back on a CRT in a dark room. On an LCD, however, the dark areas are so washed out that they may as well be gray instead. That's the issue.
Of course, different people find different things scary, but the darkness of the screen makes a huge difference for me. Horror games kind of left me behind once flat panels became the norm. My Kuro still does a damn fine job, but it isn't quite as good as a CRT for horror games. Most flat panels are a joke.
Just a pet peeve of mine as horror games were among my favorite genres, but the changes in display technology actually really killed them for me. That's why I don't care about the Silent Hill HD collection, honestly. Those games are simply not going to look any good on a flat panel due to their heavy usage of darkness to create tension.