To clear some things up, it's been almost 9 years since the game came out in 2001, and since Monolith shut down the online servers and the fan support died out, I was only left to play single-player. Therefore, the sweet memories of inter-species multiplayer is going to stay as memories, and nothing else.
Marine Campaign
The Marines campaign has you playing as Corporal Harrison of the Colonial Marines. After losing contact with a research facility, the Colonial Marines are sent to investigate. After a failed landing, the tutorial mission tasks you with turning on the power to a landing pad so the ship can land properly. The game creates the most forced situations possible, in order for you to progress through the level alone. While it does definitely build tension, it's not the first time you walk into a building and have the entrance collapse, and hearing your squadmates say "well, do it yourself, we can't do anything about it."
Throughout the first level, it's a buildup, not knowing if an Alien could be lurking around the next corner. It's a dark place, and definitely atmospheric even today. For the first half of the mission, it's meant to scare you, with motion tracker blips, areas stained with blood, etc. However, you don't fight a single enemy, until you complete the objective by restoring power. Throughout this mission, the lack of enemies is terrifying. Then after completing the objective, you then awaken the Aliens and that's when things go downhill.
The gunplay reeks of generic style. The combat is not very engaging and feels stale. This definitely hasn't aged well, but I can see how this was awesome at the beginning of the 21st century. Since you're always anticipating an enemy around the next corner, and it's supposed to be a scare tactic, is it just pointless if the combat is not fun?
The scare factor is also greatly dumbed down with the "save anywhere" save system, meaning as long as you can pause the game at any point in time, you can also save the game. It kills all tension you have because there's no worries with this kind of save method.
Basically, the campaign consists of atmospheric environments and monster closets full of dry, unsatisfying combat. The scary elements wear off a bit too quickly and you're left with a campaign that isn't very enjoyable.
However, the objectives can be quite varying, and the sound design really shines with your gunfire, motion tracker and enemy screams. The audio is also backed up by some funny banter between somewhat well voiced Marines.
Aliens Campaign
The Alien campaign chronicles the birth and evolution of an Alien.
From the first mission, you play as a wall-crawling, pouncing facehugger, who must find a human host to create more Alien lifeforms. Playing as a facehugger is unique and forces a stealthy playstyle to find one isolated human that isn't close to anyone that will kill you. If you chose the wrong host, you'll likely be torched to death and fail the mission.
When you succeed, you then play as the result of your facehugging skills, the chestburster. You start out inside the host's stomach and chew through his stomach. Awesome. Your playstyle is then changed once again, and you can play as a high jumping, slithering lifeform that cannot wall-crawl or attack by lunging. You are tasked with eating small creatures and such to survive and later evolve into an adult Alien.
As the adult Alien, the stealth elements of the game are a lot more dumbed down and is more focused on rushing in and clawing people to death. Although the variety of gameplay styles wears off at this point in the game, it is counterbalanced by the objectives because they become more varied.
Overall, the Alien campaign succeeds in creating lots of gameplay variety and is easily the most memorable campaign of the three.
Predator Campaign
The Predator campaign has you playing as a Predator that has escaped from captivity because of Corporal Harrison's actions during the Marine campaign. What is really cool about the three campaigns is that the events of each campaign overlap with each other and appear from different species perspectives.
After escaping captivity, you start off hunting humans down, with a simple pair of wristblades and a speargun, but you slowly recover your inventory to be a true hunter. Some of the Predator missions really bring out the outdoor environments and are a really nice break from a lot of indoor levels.
You are just sort of given the items back, by random Predators, and it's not very well explained. What's up with that?
Much like a Predator, you have to use stealth and tactics to effectively kill your opponents. If you fail and don't play stealthily as the game plans you to, there's always flailing your blades at the someone's face for eventual success.
In the beginning, it can be quite challenging to fight against enemies because of your lack of inventory, but later you get gadgets that heal your health and energy in between fights, making you rather unstoppable. It's essentially the opposite of any game formula, it should normally start easy and get harder as you progress, but it's the opposite case in the Predator campaign.
Last Word
I feel that Aliens vs. Predator 2 has not aged very well in the past 9 years. I want to enjoy it, but some of the aspects feel far too dated. The gameplay for the Predators and Aliens feel very different and rather fun to play as, but the AvP2 has way too many instant-death situations where if you don't play it the way it's meant to be, you'll have to reload your save. You're not that free to do what you want, and that kind of restriction is disappointing. Also, the Marine's gameplay feels incredibly stale, and this is also probably influenced because of the high standards of shooters.