Great point. The dread and anxiety in that film was not knowing and watching what "not knowing" did to those who became obsessed with the answers. The danger was close, the pattern seemingly always just coming into view, but the puzzle was never complete. We also got the sense that getting the answer would cost far too much (a la 1988'sThe Vanishing), so it made sense to pull back from the mystery. This way lies madness and be careful looking into this abyss--this one is looking back.
What NCFOM did for thrillers, this film did for whodunnits.
What NCFOM did for thrillers, this film did for whodunnits.
Also, it helped popularized that awful trend in Hollywood of replacing letters with numbers in movie titles, so that's another knock against it.