By the chronology above, I would say themes in the first are all top notch, with the only flaws being in their adaptation (the films themselves are blameless in most cases with the limitations imposed by their time). The horror movie has always been the realm of teenagers, with the common cliché of horror movies making great dates. However, the genre didn't open acknowledge this until the 90's (the b-movie entries to the genre acknowledged this much further back, but we're talking about big budget movies here) with the release of Scream. When the writer of Dawson's Creek and Vampire Diaries is writing popular horror movies, the industry is blatantly courting the teen demographic.
By its very nature, the horror movie should be the realm of adults, but this shift in focus has resulted in a dearth of serious offerings, with even paint-by-numbers entries like The Conjuring receiving almost universal critical praise even though it does absolutely nothing original. The landscape is so barren, that just by executing all the horror tropes efficiently, you become a shining example of modern horror.
The problem can be traced, as so many of modern cinema's problems can, back to the audience. Our attention span is so much shorter now, that we need the action ramped up faster than in horror's halcyon days. Think back to Alien or Invasion of the Body Snatchers (I know the picture in the header uses the 1956 version, but the 1978 remake is far superior) and how long those movies took to reveal the horror, using a large chunk of the movie just to build the ambience and attach you to the character(s).
The '00s has seen a wave of remakes as about the only new offerings to horror fans. While Hollywood loves to call these reimaginings, they tend to be just bland recyclings. Cabin in the Woods expertly (and happily) dissected the entire genre, showing just how stale it has become. But the answer to that problem is to breathe in new ideas, not abandon the genre entirely. Filmmakers like Guillermo del Toro, who frequently add horror elements to their movies in other genres (Hellboy, Pan's Labyrinth), but still produce fantastic horror films (The Orphanage) show that we haven't exhausted it yet.
The recent move towards serializing horror on television is intriguing, if a bit clumsy in its initial wave. "American Horror Story" moves between mediocre and awful but perhaps FX will apply the lessons learned to del Toro's own upcoming horror series "The Strain". It is at least a new opportunity for a genre in desperate need of new ideas and a second chance.