I can't think of a single thing that's wrong with this game. Even Half-Life (both 1 & 2) can't achieve this level of absolute perfection. While the gameplay mechanics have mostly been recycled from the original Halo (and that's a good thing -- after all, it's not a very good choice to try to fix something that isn't broken, only to break it anyways, like many developers have done in the past with sequels), Halo 2 does add some new tricks to the already excellent formula to create perfect gameplay. You are a cyborg named Spartan 117 (AKA The Master Chief) that was one of many Spartan cyborgs that were built by Humanity in an attempt to win a losing battle against an evil alien race known as the Covenant. And I mean "evil" -- these aliens just started a war against the humans because they thought that humanity was an affront to their gods. All the other Spartan cyborgs were blown up by the Covenant while they were being cryogenically frozen. In fact, you are the only Spartan cyborg left, and Humanity's only hope. People who have played first-person shooters for a long time may not notice anything new in Halo 2's storyline, but who cares? There's probably no other storyline to use for a futuristic, sci-fi themed game that takes place in deep space.
Gamers have been sick of always playing as some kind of "tough guy" who can take 40 bullets to the head before dying. Yet, when games like Splinter Cell incorporate a "one-shot, one-kill" rule into the gameplay, they become frustrating for unexperienced players. While Spartan 117 can only take a shot or two before dying, he has shields that protect his body from damage. So, technically, you are another "tough guy" in Halo 2, but there's a realistic-enough excuse for it. Since your shields recharge to 100% a few seconds after you've stopped taking damage, there are no "med-kits" in the game, and there doesn't need to be any med-kits, either, since your shields recharge like that. This is a superb gameplay mechanic -- I haven't been sick of playing as a tough guy, but I've been sick of when you waste a med-kit or other object because, let's say, you had 99% of health and the medkit only restored 1% of health because it didn't want your health to go over 100, but you couldn't use the medkit again to restore 24% more health. What annoys me just as much is when you make a little mistake (like taking damage from a fall) and have to backtrack to find a medkit to restore your health. With the shields system, restoring your shields is as easy as them automatically recharging to 100% a few seconds after you made that mistake, and you also don't have to worry about wasting a medikit just to restore 1% of health.
Adding to the realism, Spartan 117 can only carry two guns at once. They don't have to be a specific kind of gun, they can both be anything you want to carry. If you want to carry another gun and already have two other guns, you must discard one of your weapons to make room for that weapon. I mean, Gordon Freeman from Half-Life can carry over 20 guns at once, a cliché carried over from the earliest days of first-person gaming, and that's just unrealistic. Another cliché that was carried over from the earliest days of first-person gaming concerned melee fighting. You could only use one weapon for melee fighting, which was usually a knife or brass knuckles or a sword, and they packed much less of a punch than in gun-based combat. Did someone not realize that you could swing the guns themselves at opponents, which packs MORE of a punch than in gun-based combat? Apparently, the people at Bungie realize that very well, because that's what you do in Halo 2 for melee combat. And that's realistic. Spartan 117 can't sprint, but since he moves at a fast-enough pace just walking, you don't need to sprint. Grenades are not handled as a separate weapon in Halo 2 -- rather, you store them in your left pocket to throw simply by pressing the left trigger, which is realistic, as opposed to having to switch to a separate weapon to throw grenades (as in Doom 3).
You simply pick objects up by walking over them. While this may be the only thing that's not realistic, it is appropriate for the fast pace of the game, where it would be somewhat frustrating to have to manually pick an object up in the heat of battle. The only exception to this is when you swap one of your weapons for another weapon. After all, nothing is more annoying than having the game automatically swap a weapon when you didn't want to swap it, or if it swapped the weapon you wanted to keep. In Halo 2, you press the action button to swap weapons. This allows you to switch to the weapon you want to swap, and decide if you want to swap that weapon for the new weapon you'll get. The game will tell you via a message at the top of the screen if an object can be used with the action button, so that you know what you're doing.
Such excellent attention to realism means that you will be immersed into Halo 2 for a long, long time. Other excellent features include some of the best graphics, audio, physics, and AI ever in a video game (The graphics are so good, I almost threw up when I started riding an elevator in the game). Only one thing is certain: If you don't play Halo 2, or if you don't like it, then there is something wrong with you. A definite 10 / 10 to this one!
