I think that the writers hoped that by stringing together various scenes from the original Predator it would somehow be a good film, when in actual fact it’s really quite boring. The initial premise is interesting enough; the "heroes" having been dropped from a plane while unconscious have to come together and figure out what's going on. Of course they're being hunted by Predators who obviously enjoy hunting humans as much as Aliens.
Everything is going quite well (in terms of the film, not for the humans), until they find Laurence Fishburne in his hideout. At this point the film loses all momentum and just becomes boring, the balance of the film shambling from one encounter with a Predator to the next. There is almost a bit of a twist towards the end, but then there isn't, and tedious parity is restored. Compare this with the superb script in Predator, which manages to introduce the characters so well (almost as good as Aliens), whereas I don't feel that we really got to grips with any of the cast of Predators.
As well as bad writing, Adrien Brody is the wrong actor to lead this film. I didn't think he was an action star in King Kong, and I don't think he is here either. Putting on a gruff voice and working out for six months doesn't make you an action hero. Laurence Fishburne might have been able to carry the film, but I doubt he would have wanted to. Like I say, Predators just tries to regurgitate what was good about Predator: rehashing Alan Silvestri's superb score from the first film, same setting, a guy with a mini-gun, falling off a cliff into water, a guy who turns round to face a Predator with his shirt off, even using Little Richard over the closing credits. However, this can't stop Predators being average at best.