Caution: Long post. I wrote this out last night because someone asked me what I thought
At first, as with most games, I was kind of dazed. I had no idea what was going on. I was having trouble getting used to everything -- no customs for me to mess around in. However, I got off for about an hour and a half after playing on every map at least once, then got on and started with the ranked games.
First of all... the game is insanely balanced in it's normal state. Most times when I thought about it even possibly being unbalanced, it was just stupidity on my part, or the parts of others. I was on a 19 kill killing spree as me and a team mate were driving around in the warthog, myself as gunner. Unbalanced? No way. First of all, the other team kept jumping out into the open, fault one. Also, nobody really knew where the spartan laser on Valhala was, so that was fault two. Once they finally figured out that if they were to stay in close quarters, they took me down quite easily. Not only this, but it's fairly easy to come back from a -20 loss and tie, and even win. Power weapons aren't as powerful and important as they were, but not so much so that they're useless.
The Spartan laser. While a lot of people have said it looks horribly unbalanced, it's really not. It's not the easiest thing to aim, it's all timing, and that timing is very sensitive. The plasma pistol. My last game tonight, I got plasma pistoled twice. I got into my normal "plasma pistol whore" mindset when I realized... I was standing virtually still in a close quarters when I got hit. Out in the open it was almost completely useless. Spike grenades. Said before, I'll say it again. Nothing is more satisfying that sticking a person with it. They're SO much more fun than plasmas.
Assault riffle. Since I really didn't play that much Halo 1 multiplayer (I only got Halo 1 about a month and a half before Halo 2 came out, got it with my Xbox), I never got to feel how amazing this weapon was. I remember it vaguely from the campaign, but it was soon over-shadowed by the SMG's. SMG's were alright.. I was much more of a "Go to a dual wield instead of the BR" kind of player in Halo 2. But.. the AR is a perfect starting weapon. It makes you question if you want to get rid of it, and it's good for just being there. Honestly, I think the clip is a little shallow. A few more shots is usually all I need to finish someone off, but I believe that's just my misuse of my ammo (firing at walls when I should stop).
Graphics. They look great. Not "wow, it's so lifelike, you sure this isn't a live action movie" great, but Halo 3 great. Bungie, in my opinion, has so far nailed the perfect mix of next gen graphical additions (everything is so glowy!) while still keeping that familure Halo feel (how much can you incrase the character model of a Spartan, really?). Water is very nice to look at and to shoot at. Walking backwards produces wake and splashes. Walking in snow creates footprints. But, again, I must be honest... the water feels a bit "springy". It could do with more splash when driving through it, also. Otherwise, though, it's stunning.
One last thing for graphics -- atmospheric effects. I remember reading somewhere how Bungie really didn't want to place them in, but I think they would change the game for the better. High Ground, for example. Imagine if there were various densities of sand that blew through the map, reducing visibility. Snowbound, also, could do with snow drifting through the map reducing visibility sometimes. Maybe not all the time, maybe even a custom option. But, I think it would really make it pop.
Audio. I don't have a 5.1 setup (though, admittedly, I am going to try and set it up before the Halo 3 release), but the audio is amazing. Far off fights have that "far off" sound. Not just reduced quality, but the sound it would make as if it were actually having to travel through various terrains, objects, and the atmosphere. I love the sounds the weapons make... even reloading the AR. I also found it quite trippy how being under water muffles everything as if you were actually under water.
Saved Films. Amazing idea. I can't wait for the full setup... several things now I've done I wish I could view from the point of view of the enemy, and from different perspectives. Fast forward, rewind, etc. I stuck a guy with a spike grenade from behind the gate in High Ground, blind throw. I wish I could see that.
Thar. </end long geeky post>