In weighing and determining the best horror films of all time, there are many important factors to consider. Among those:
1. Was the film scary at the time of the original release, and does it continue to be scary today (in other words, does it stand the test of time?) Gazing at the list quickly, I would judge that Halloween, Blair Witch Project, The Haunting and many others meet that benchmark with flying colors in 2008, even years and in some cases, many decades after the initial theatrical release. I know that I steadfastly refuse to watch either Halloween or The Exorcist when I'm alone in the house.
2. Can the film in question be interpreted in more than one way, meaning that -- again -- it survives beyond the original context or time period? If you study a few examples, say Halloween, The Blair Witch Project, Rosemary's Baby or The Texas Chain Saw Massacre (original), you can see that they all spur various interpretations. Is Chain Saw merely surreal savage cinema, or a statement on vegetarianism? In Halloween, is Michael Myers the Bogeyman, or physical externalization of Laurie Strode's repressed id? Is there any witch present at all in The Blair Witch Project? Alternate readings of these films mean that they are more than merely scary...they are timeless. They float above the original context and become...universal.
3. A great horror film not only reveals something important (social or economic) about the context in which it was created, it can actually come to embody that time period...and become a touchstone. Consider Night of the Living Dead (1968), which perfectly captures the Vietnam War Age, or Cronenberg's The Fly, which was released just as America's awareness of AIDS was growing and taking shape. You can't talk about horrors of the 1960s or 1980s without mentioning these films.
My point, I suppose, is simply that the horror films of relatively recent vintage may or may not stand the test of time. We just don't know yet. Therefore, to praise them on points 1, 2, 3 and is, perhaps, premature. I have very, very high regard for recent examples of the genre including The Ring, Hostel, Silent Hill, The Descent, The Strangers, Cloverfield, Vacancy...even, to some degree, The Ruins.
But since we are still locked in the 2000s; since we are still entrenched in the Bush Era Mindset (at least for a few months...), it's near-impossible to stand back and objectively look at these films as touchstones or time-capsules of the era. Because we don't really know, ultimately, what we will carry out of this turbulent era. Maybe it'll be Snakes on a Plane. We need distance to rightly assess these recent works beyond the value of these questions: "does it scare me now?"; and does it relate to how I see the "today" right now?
The fact of the matter is that time can also wash away the tidal wave of bad reviews for fascinating, individual, quirky, yet much-maligned films like Ghosts of Mars, The Happening or X-Files: I Want to Believe. It is entirely possible that is one of those films -- the ones which most critics hated or missed the boat on (historically, think The Exorcist...) -- that will emerge as the "new classic" of the age. Perhaps the real value of these films will be excavated. Who knows, perhaps the new classic horror film is the very one I missed the boat on, such as Diary of the Dead, or High Tension.
Only time will tell...and that's the reason those films didn't make the list, in my opinion. It's just too soon.