Look, I'm a horror fan, and if you are going to watch this film, you certainly are, too. And I absolutely LOVED this film.
Sure, it's a story about partying teenagers who transgress in the woods and then get picked off by a masked stranger. That's not "lack of innovation", that's a SUBGENRE, which this film totally nails.
Yes, it's not until the halfway point that "the action" begins. But I consider that a pro not a con. I actually enjoyed getting to know the characters, who despite being high school archtypes, are more real and fleshed out than their counterparts in American films, whom often seem as though they are designed for us to detest them so that we don't feel as bad when they are killed.
And the movie makes up for the lost time in the second half.
I appreciate that the characters act in ways that seem appropriate to who they are, rather than just in ways that will further the plot. That was accomplished by giving us time to know who they were.
I also loved the fact that the killer didn't simply skulk about in the shadows picking victims off one by one; he was NOT afraid to mix it up! While he was certainly...effective, he was not superhuman.
If you don't like this film, you either don't like slashers or you are too jaded to appreciate them any longer.