There are very few things in life that are more annoying than feeling like you've gotten the old bait and switch. That's kind of what this movie delivers. I realize it's not the movie's fault that IMDb has "horror" as a tag for this movie, but still...no. No to this being in any way a horror movie. I think calling it a thriller is even a bit of a stretch. There's a few tense moments, but on the whole it's just a drama about two people who can't stand each other.
The premise of the movie follows a husband and wife over a long weekend getaway to the beach in Australia. The problem is that the friends they are trying to meet up with never arrive. Oh...and they hate each other. That too. The whole running thread of them constantly pissing off nature is hardly even a subplot, but somehow manages to be the impetus of all the bad things that happen. I get it...it's bad to litter and be a general prick towards nature. I wholeheartedly agree. But then again I seriously doubt nature has time to waste being a vengeful jackass to two small time folks when there are...say...millions of other people harming the environment in much more serious ways. But I guess that's the point (maybe); if John Everyman isn't kind to nature, nature will quite literally kill him. Or something. Regardless, Long Weekend manages to be middle of the road at best in just about every aspect. There's nothing here I'd even remotely call horror, it fails to foster any sort of paranoia or even cheap jumpy thrills like a good thrill, and pretty much everyone involved is thoroughly unlikeable.
That's pretty much how I would sum up the whole movie...thoroughly unlikeable. Which is really a shame since I really like some of Jim Caveizel's (or however you spell his last name) other work. I wouldn't say you should avoid this movie, but there's certainly better movies with similar themes.