A significant improvement, but still not impressive when stacked up against its PC competition.
Halo 2 is set almost immediately after the original game, with Master Chief back on Earth along with Sergeant Johnson. Who is Johnson you ask? I have no idea, the story says he was with Master Chief on Halo. There were a few individuals who looked and sounded like him in the original game, but they had no names, and there were supposedly no survivors aside from Master Chief according to the ending, so I have no idea where he came from. Anyway, the big tag line of Halo 2 involved Earth being invaded... and yet the Earth invasion only takes up the first hour and a half of gameplay. The storyline mostly centers around a second Halo, the Flood, and those fruity religious aliens. Halo 2 tries to bring the storyline up to a more complex level in the sequel, but you can tell that the writers are amateurs since there are times when it feels like they have no idea where they're going with the story.
You not only assume the role of the Master Chief in this sequel, but you also get to play as a Covenant Elite called "The Arbiter" who was disgraced by the destruction of the original Halo. This sounds good in theory I'm sure, but aside from story and who attacks you, both characters play exactly the same. They jump the same height, run at the same speed, and can make use of every single weapon and vehicle they come across. Is this bad? No, but it's not good either... if you're going to include multiple playable characters at least make them different in areas other than story.
Halo 2 doesn't depart much from the gameplay mechanics of its predecessor. You can only use two weapons at a time, you can use a melee key\button to beat enemies to death, and you can drive vehicles. There have been some changes, one is the removal of health bars. Once your shield is gone it only takes a couple more shots to kill you, which in my opinion was a fairly foolish design decision since there is often a lot more enemies at once in Halo 2 than there were in the original game. Another major change, and a good one, is destroyable vehicles. In the original Halo you could drive off of cliffs, and get blown into oblivion by three tanks at once and your vehicle would be unharmed. In Halo 2, Vehicles take damage and can be destroyed if they take enough.
One of my biggest complaints with Halo: CE was the poor level design. Halo 2 is somewhat improved in this area. There was a lot of copy and pasting in the original game after the first surface mission, but in Halo 2 there are only several places where areas are copied and pasted and they are far between which is nice. Still, I'd prefer it if they were not present at all. The level design is still far from above average, most of the rooms are fairly bland in appearance, and often you'll find yourself trapped in a room where you have to fight off wave after wave of enemies until a door magically unlocks, or a wall conveniently explodes allowing you to progress. Great, an arcade shooter. Still, the level design isn't all bad, there were some moments of brilliance that took me back to my Unreal days. The levels where Master Chief first lands on the new Halo were very nice, unfortunately the rest of the game does not feature the same design quality.
Obviously Halo 2 is dated, the game is several years old now, and it was created as a title for the original XBox. Still, the graphics aren't too bad and can be nice to look at if you don't mind the fact that a lot of it looks very cartoonish, and like it was taken out of a comic book just like the original game.
The audio is great, you've got solid voice acting, atmospheric sound effects, and like the original Halo a great musical score. I still find the English speaking alien thing a little hard to swallow, but every Sci-Fi game that includes aliens does that, anyway, so its hardly a flaw.
Surprisingly, Halo 2 is a vast improvement over its predecessor. I don't consider it the greatest FPS ever, or even revolutionary for that matter, but the campaign is more enjoyable than the original's, and it feels more polished overall. Still, the "fun" multiplayer is little more than average, I'd rather play something like Unreal Tournament since that is actually fun to play. Regardless, when Halo 3 is inevitably ported to the PC I just might try it. I still don't see what all of the hype is about, though, while Halo was below average, Halo 2 is simply average, am I playing the same game as everyone else?