Two first person shooters, as different as night and day. These games are true blue science fiction, taking the next era of mankind, setting up conflict with an outside force, and setting you free to shoot thousands of aliens.
I would say that Halo's an optimistic sci-fi universe, where we managed to get our society's act together enough to be colonizing other planets, have bullet trains with stops in Manhattan and Harvard Station... the government does have some shady dealings involving shanghai-ing promising young children into the super-soldier program and turning volunteers into AIs, but all in all it's a good life for people. Oh, except for the poor people who don't even know what kinds of medical testing their children are legally entitled to if they are at risk for a disease... and the fact it's a totally surveillant society; if you're "off the grid" you have a very hard time doing anything.
Anyway, that was the Halo universe until the Covenant came along and gave everyone the common enemy. (Unless it was the Covenant that inspired the government to turn up the heat on their Spartan program and start getting more than just volunteers into it.) In general, it seems to me that the Earth of Halo is a logical extension of where we are now; government gets more intrusive, the class division gets greater, and technology makes the small things in life better.
You play a military man, the famous Master Chief, product of the Spartan program, uber-soldier. There is danger here, but it's all part of the job. As part of the military, you are often backed up by a squad of marines - something Bungie has gotten right since back in the days of Marathon.
Half-Life, on the other hand, is the story of an Earth gone horribly wrong, when the Black Mesa research facility opens a portal into another dimension while attempting experiments in teleportation. You spend the duration of the first game witnessing aberrations, saving scientists, and dodging the marines there to shut the whole operation down... trying to escape with your life. I'm not quite sure how the game ends, actually, as the last level was incredibly annoying and I figured there wasn't much plot left to be had. I suppose I could have been wrong; my assumption that you exit the other dimension and save the day seems to have been wrong, since...
Fast forward to Half-Life 2. The world of Half-Life 2 is one of alien overlords. The "benevolent" overseers have been showing us the way, teaching us that our instincts are the enemy. There are shock troops everywhere, and mounted high in the train station is the Big Brother style image of the old site administrator from the original Half-Life, spouting propaganda about how good it is the aliens have the fields up preventing the propagation of the human species, and promising that once we are "ready", the fields will be turned off. There are stormtrooper-like police everywhere, and they pull you aside at the customs/security-like entryway to the city. Seems you're a bit of a celebrity for your escapades in Black Mesa, and it's not long until you're on foot, unarmed and running. Floating droids take your picture, people all have that hollow-eyed expression of doom (note: faces in this game are the best I have ever seen in a game.), and when you find yourself in some people's apartment, they are watching television - the same images you were seeing in the train station. There's only the one channel.
It's a completely horrific setting, and the palpable sense of oppression keeps my sessions playing this one short. But I do keep on coming back. A lot. Running through the canals of the city, hiding behind scrap metal and meeting the dregs of this new society, these underground railroaders are doing the most important job of their lives helping you evade capture. The colors are all the colors of a city; smoggy brown, grey, metallic.
Half-Life and Halo both use the external enemy to avoid having the player kill humans, or really, have any moral ambiguity about his position as the savior of mankind. You kill these aliens because they are zealots; they are the ones who have made the "us-or-them" decision, and you're just helping them choose wisely.
The character of the danger in the early stages of Half-Life 2 is distinct from that in Halo 2, in that you feel vulnerable. At the beginning of the game, you don't even have the suit to protect you, and you really are just walking around in street clothes, waiting for a sadistic guard to randomly choose you to beat down. In Halo, you're pretty much always suited up and ready for combat; sure, you start out the first game unarmed, but you are most definitely armored, and the scenario seems tailored to the fact that you are unarmed; you watch combat at a distance, and go through once it has died down. There's never a sense that you could just up and die, if you took a wrong turn.
The controls affect the character of the game, too. In Halo 2, with the XBox controller, there's only so fast you can turn around, so they avoid making situations where that's absolutely necessary. This basically translates int no surprise attacks from behind. With Half-Life 2's controls, you can turn around very quickly (as quickly as an unarmored person could, you might say)... so there are times when you're looking around, trying to figure out where you're getting shot from. Aha - this touches on another difference. In Halo 2, the aliens are using energy weapons. With glowy bolts of energy. That conveniently trace back to where they are shooting you from. Half-Life 2 has good ol-fashioned projectile weapons, making it a bit harder to spot the subtler trails of glowing hot lead flying at you at supersonic speeds... And in Half-Life 2 it's a lot more important you track them down fast and find cover, since you're a lot easier to kill.
These factors do not make one game superior to the other; Halo 2's confidence and tactics just make for a different game from Half-Life 2's fear and jumpiness. They're two different games with the same basic grounding - the variation available in the first-person shooter genre is quite striking.
Oh, and whatever happened to Doom 3? I guess I should talk about that some too; these are the three FPSes people were awaiting this year...
Doom 3's got a different kind of dystopia, one where corporate interest roam free of moral constraint on the surface of Mars, doing research into all manner of ... Actually, now that I think about it, there's not all that much difference between the site in Doom 3, and the Black Mesa installation in the original Half-Life. Experiments in teleportation let hellish creatures (actually from Hell itself, in Doom 3) into our dimension, and it's up to you to stop them. The main differences are in the engine and, generally, the graphic design. And the fact I need a CD in the drive to play Doom 3.
I'll figure out where it fits into this later on.
Or probably not.