Let's say that the cardinal example of the slasher film is Alfred Hitchcock's "Psycho" made in 1960. It was the first horror movie to really utilize all of the most essential tropes; a beautiful and sexually active woman protgonist, a killer from a troubled family, the scene is somewhere unknown, away from home, the attack is personal (without a gun), and the murder i seen from the victim's point of view.

I have always seen horror movies from this sort of vantage point, using "Psycho" as a guide for good horror.
The horror movies that I watched growing up and today seem to really play up the sex and violence aspects, and the relation between horror and porn. Low budget horror film has been around since the 50's and 60's with B-list directors like Ed Wood and Roger Corman making light sexploitation movies for the drive-in teen boy crowd. In the 80's there was a wave of low-budget, underground horror films, which became known as "video nasties" and were crusaded against by the English and American governments. These films were more closely related to porn than most mainstream horror of the day, from there production level, to their marketing campaigns (in trashy magazines) and their distribution. I think this type of underground cinema really changed the way modern horror movies look and feel.
1. There is a lot more sex in these movies.

Like the sexy girl above from "Texas Chainsaw Massacre" who is molested by the demented family of killers, women are often sexually abused, or mistreated, but are never really 'raped', because that would be crossing the boundary. Also, most of the omen in these movies are shown to be quite promiscuous, and in many cases their fooling around leads to their untimely death.
2. The graphic scenes are more realistic.

Like the scene above from "Final Destination", graphic violence has become increasingly more violent and gorey. With slasher flicks becoming more popular and higher box office grossers, they get larger budgets, resulting in the art director being able to experiment with gore a lot more than in the past. Also, there is a much larger work pool for special effects people now, it is not as specialized as it used to be and prosthetics are less expensive. We live in a time and country of brutal violence; there is extreme violence happening all around us from crime to war. We crave weirder and harsher violence in movies as a way to combat our own fears towards these real life issues.
Because of these problems, along with many others, horror movies have both become more popular among teenagersa and specific niches, while also being harshly critcized by ratings and government agencies. As a person who was deeply afraid of and tormented by these types of movies as a youth, I am just now trying to deal with my fear and address all of thesse issues inherent in horror. However, I don't think most teen boys going to see these movies really examine these issues, and that's where the problem lie.