Again defining the genre in a simple sentence is difficult, but an easy way to tell a slasher film from a traditional horror is it is much more focused on the villain, either showing trace parts of him everywhere, or upright showing him killing people. Good ones will do more of the former and less of the latter. A slash will also have gory deaths, or at the very least very intense deaths. A perfect example of intense but not gory deaths is in both Texas Chainsaw Massacre and the original Halloween. Even some of the slasher icons actually come from films which were very rooted in traditional horror, which is where the line blurs, thus, the sub-genre. Also Leatherface, very different type of horror from most of the other iconics.
Many modern, and by modern I mean the last decade or so, have focused less on these classic or identifiable villains, with Ghostface and Jigsaw being some of the very select exceptions. Now horror films focus on a much more human enemy. By the middle of most of the horror giants franchises they had become unstoppable killing machines, capable of mowing through armies of sex-crazed teens. With movies like I Know What You Did Last Summer these villains have become much less memorable, and much more killable, despite coming back for like 4 sequels.
Some slasher-esque films even go with concepts for their villain, for example the Final Destination series, or I suppose the Happening, even if it failed miserably in execution. A good iconic villain has simply not been created for a long time, and I'm not quite sure why that is. There are still creative people out there, and infinite ideas to work with, but maybe the classic image of a masked man slowly and menacingly walking towards the protagonist is simply not scary anymore. Nowadays psychological horror is bigger, and slashers have been way on the down low, despite decent profits from the recent remakes.
That's another thing I don't really understand, why is it that remakes never capture the 'magic' of the originals. For the three main horror legends, Freddy, Jason, and Micheal the remakes have failed by showing them more, or by splattering as much gore as possible on the camera and just praying that it is scary. It's not guys, gore is not scary, you need something scary behind it. Behind you. You know why Jaws was effective? You didn't see the shark, like at all. Maybe five minutes in total, and that's stretching it. The unknown is really fucking scary, but what is even more scary is the little known, the hints of whats to come.
Unfortunately many films go to shit during the conclusion, stretching on for too long or becoming too comical, or maybe making the villain too human, or too inhuman. It is a very delicate balance that seems almost impossible to get right, in that even my favorite slashers conclusions are somewhat lackluster. What I'm saying is that even the best of them simply do not know how to finish, because conclusions are difficult when you want the badass you created to be defeated, but not utterly, and in no way humiliated. To die with dignity is all you want, and sometimes you can't even get that. Concluded next week.