There will be minor spoilers, but nothing terrible unpredictable.
The film starts with people falling out of the sky and parachutes deploying in order to see them (more or less) safely to the ground. We gradually meet the full lineup of characters and find out more about all of them. Basically, everyone is armed (from a Shiv to a Minigun) and trying to figure out just what is going on, and it takes them a little while to figure out that they are in fact on another planet. The hero is played by Adrien Brody and he is packing an AA-12 with a Drum Magazine. The first guy he meets is a Mexican (played by Danny Trejo) carrying a pair of pistols and SMGs (MP5s IIRC). They are interrupted by a Russian with the aforementioned minigun. While they are talking, the woman of the film shows up with a sniper rifle pointed at them. Then they wander over and meet someone who was on the FBI’s most wanted list (complete with orange jump suit and shiv) and a member of the RUF (who gets an AK-47). Finally, they find Topher Grace in what appears to be yet another role he really shouldn’t have. Seriously, which genius thought he would have been a good Eddie Brock/Venom in Spider-man 3? This time he is a doctor and only has a scalpel with him.
Anyway, eventually they meet up with a member of the Yakuza (complete with pistol), and that completes our merry band of adventurers.
There are numerous homages to the previous films. Including Brody getting covered in mud and shouting “Kill me! Kill me now!” Also, at one point it looked like one of the corpses that the group came across had had an Alien inside him, specifically, it looks like something had burst out of his chest, rather than someone caving it in to rip out his heart. In Predator 2, when Danny Glover goes onto their ship, we see an Alien skull mounted near the rest of the trophies, so maybe this was their subtler way of showing off that relationship, as the killing of an Alien (and taking a trophy) was the highest honor in the Predator society (or so the comics have shown).