Ok.. well since people are interested, this game kicks Halo's ass. I have not been able to play as much as I would of liked but my initial impression are good. Im assuming it is assumed all info provided will be spoiler information, so if you dont have any interest dont read below..
While you play through the campaign you play through as the covanant as well as the cheif. One of the coolest perks of this is the ability to camo for short periods of time at the push of a button... and you start with the energy sword.
The only thing I have about the sword thus far, is that in the campaign they have energy that gets used up. I didn't really like this but it works out ok, because they are fairly plentiful. However, in multiplayer, the sword does not have expendable energy so you can go slash happy all you want.
The Multiplayer Maps (and to most people these are most important) are very well done. Some of the levels are huge though and playing with 1 or 2 other people on my box couldn't give me a complete idea of how they would be with larger groups. Personally I didn't really enjoy multiplayer unless it was 6+ people or Co-op (in the first game). And since someone did ask, yes, there is co-op play in halo 2.
Ghosts have had some changes as well. Most of you have probably all seen the video with the multiplayer play that bungie presents. Personally I think the changes were pretty good, I would call ghosts a little more balanced now. First off, in standard use (without boosting) they are slower than before and are pretty easily avoidable, however if you get caught in their fire (which is at least twice the firing rate of the first game) you will die pretty quickly. When you do boost, you get movin pretty quick but lose a lot of mobility in the turning department and you cannot fire. Also note that you cannot kill someone just by tapping them with a vehicle, you need to have the right speed or they just go bouncing off with some shields gone.
While on that note, there i no health in the game, just shields. But you can take a few hits when the shields are gone and they recharge a little faster. But there is a little more strategy involved because running into a group of enemies and expecting to smack them all isn't going to work in this one. They can mess you up real fast given the opportunity.
AI has been improved a lot, I dont recommend playing on legendary the firs time around... really. AI seems to improve a lot as well as accuracy with the difficulty. On legendary they will pop you in the face every time.
There is quite a variety of weapons to choose from this time around, slighty more than I thought there would be. There is pretty much a version of every gun for the Marines and for the covenant, though each has their diffrences. One thing I noticed that doesn't seem to be true, like they showed in the demo, is the ability to do single shots with the battle rifle. As of now (at least in the french version) you fire 3 round bursts every shot regardles of being zoomed in or not. Ability to lock on with the rocket launcher is awsome. Plasma rifles also fire 3 shot bursts and their rate of fire has increased and gets even faster if you hold it down for a second or 2 (duel weild 2 of these and they decimate alien scum). There is also a Covenant version of the sniper rifle, the Beam Rifle, that is pretty sweet. It overheats very fast though so you can only get 2 shots off before you need to let it cool down if you are firing fast. If you get the timing down you can fire fairly quickly without it overheating. I dont want to reveal all of the weapons but there are some other new ones that are pretty cool. Oh, and there is kickback with the SMG so you need to compensate.
Overall appearence of the game is amazing. Its not exactly the "realism" that you have with Half-life 2 but I dont think they wanted that. It still has a Haloish feel to it that I really like, with some amazing textures. Plasma's have a new look to them when they blow up that is very nice too.
Thats all I can really think of at the moment and I'm at work.. but if people have any specific questions feel free to ask.