I know some people were genuinely spooked by The Blair Witch Project, believing it to be "real" found footage, but according to my admittedly unscientific investigations, for every person that was scared there were three people who were irritated by the horrible "improvised" performances and/or nauseated by the 'shaky cam' photography. The film's incredible financial success -- especially compared to its tiny budget -- spawned dozens of (creatively) cheap imitations, like a copy of a copy (with apologies to Multiplicity). Frankly, if I never see another poorly-made 'shaky cam' horror film I will be quite happy. And I reserve a special place in Cinematic Hell for crap masquerading as "the real thing" just to try and shake a few coins loose from horror junkies like myself.
The third point -- "not showing everything" -- actually hearkens back to "pure classical horror," as acknowledged by Paranormal Activity co-producer Steven Jay Schneider. He doesn't list titles, but the classics must include the Universal Studios monster flicks of the 1930s and the superb, atmospheric pictures Val Lewton produced for RKO in the 1940s. Of course, Steven Spielberg's Jaws is probably a bigger influence on today's younger filmmakers as far as creating suspense without showing everything (right away); Cloverfield director Matt Reeves directly references him in the article.
I admire filmmakers who try to be inventive, so I hope Cloverfield and Paranormal Activity turn out to be good, suspenseful pictures. But an interesting story, characters I care about, and a dark atmosphere are what make things genuinely creepy in my ideal horror playbook, not big-budget studio productions pretending to be homemade indies or homemade indies pretending to be "real."