What my interpretation of what some folks seem to have an issue with is that the game does not play like a standard first person shooter. The others did. The only real bizarre things with the other games were that the alien could walk on almost every surface and the predator could cloak and detect prey, and had homing weapons (though even these were done in other games like Dark Forces 2 prior to AvP '99). As far as the mechanics of the fighting went however it was your standard FPS fare, done up relatively well. What it mostly had going for it were creatures we are all undoubtedly fans of, and also the cold dark horror atmosphere of the games.
With AvP3, what we've run into is a game that doesn't play like completely like a first person shooter. In fact, only about a third to a half of the game plays like one. What I'd describe it as would be more of a first person action game with shooter elements and (as one poster said) brawler elements, and it certainly takes people out of their comfort zones of simple point and shoot. There's less Quake style gameplay (which I do love, this isn't a bash) since the game is more about engagements, and the engagements themselves have quite a skill curve. Some people get frustrated because they feel too locked in, which I don't blame them for because they are being locked in with their opponent. You're not supposed to be able to run away, you're supposed to finish what you started, or approach your targets more carefully. The downside to this system is that yes, one little slip up means death. The upside to it though is that your confrontations with the enemy are all that much more personal and visceral. You're not slicing at the air in front of you with an enemy just happening to be in the way, you're deliberately jumping into and facing your foe and cutting them a new hole.
AvP3 is my favorite first person game because it makes me feel on edge, worried about low clips or whether those guns can see me from my perch or if that lone warrior on the other side of that wall is prime for the picking. Each of the races plays completely differently, and the game itself doesn't play like any other games on the market. Just about everything else is either a derivative of Quake, Halo or Gears of War (or combinations of the three), which are all fun games in their own right, and I don't think less of folks who are fans of any of those gameplay styles. I've just got a preference to this little number.
Just sayin', no need to hate on the game because it didn't fill your shoe size. It certainly meets/exceeds others' expectations, and it sure ain't a bad game. It's just wildly different is all. The reviews certainly reflect this, what with them ranging from 5/6's to 8/9's. Nobody knows what to really make of it.