Short but sweet campaign, functional but irritating map editor, and short-lived multiplayer.
To start out I think that the campaign is the best the game has to offer. Several linear levels of shoot-the-bad-guys action with little mini-bosses along the way in the form of brute chieftains and scarabs, a kind of giant walking tank. It has had me coming back for awhile for when none of my friends are on, despite the repetitiveness. I don't see where people say the campaign is too short because it kept me occupied for a good 10 hours, which is about right if you play the game in one sitting (much like I do.) The only real letdown was the cruddy boss fight at the end which would have been better shown as a cutscene.
The Forge map editor is another gem in the game, unpolished, but it works well enough for the novice player and I understand that there are some players that can do amazing things with it, for example a mini-fountain of crates. It has its irritations which can turn away more volatile people. For example, there is no option or button to freeze things in the air. You have to either build up a support, put the item in question on the tower, then delete the tower. This only works on scenery items, though. There is also an irritating tendency for walls to not stay straight up, leading to map designers spending countless minutes (or hours, if you are really meticulous, like me) making sure that the walls are straight up. There is another community fix for this, which is placing a box on the ground and then pushing the wall up to it to make sure it stays up, but then you have to make sure the box is correctly aligned too. Another popular thing to do is to glitch scenery together for various reasons, accomplished by a long explanation I will not type here. Do you see a pattern emerging here? Several functions of the forge could have been coded into it, but Bungie decided to leave it unfinished. I know it can me done, because a popular mod for the source engine allows these three things. If it is a limitation of the game engine or the difficulty of coding it, perhaps it is time to go back to the drawing board.
Now then, onto multiplayer. Halo 3 has been praised for having one of the best multiplayer experiences, and is indeed one of the most popular. I, too was enticed by it and had much fun playing it for a long time, but that was before I noticed many flaws. The most prominent "feature" is that the game gives everyone aim assist. If you are in range of a player and are aiming at him, the game will slightly keep up with whatever you are aiming at to an extent. The game helps you when you are missing.
Next, the most unbalanced gun in the game: The Battle Rifle. This weapon is all range and in the hands of an average player, can headshot kill anyone in approximately 2-3 seconds depending upon the aim assist and the player's skill. I would have no problem with this if the game had no aim assist, leaving the kill entirely in the hands of the player. Anyone caught unaware by a BRing player has little time to find cover or die.
This leads me to my next grievance, grenades. While they work perfectly, they are the single most spammed weapon in the game. For example, say you did find cover after being surprised by the BRing player, well all he has to do is throw a grenade and you're done for. If you leave, he'll just shoot you. Either way you die. Grenade spam becomes most prominent in what passes for close-quarters maps in which the general tactic is to stand in one room with your team, watch your radar, spam grenades at any dot(s) on your radar, and BR whatever is left in headshot kills.
Vehicles are more balanced, as it is more logical to die in 2 seconds from a gatling gun turret than a hand held rifle (in a game, anyway.) There are plenty of counters to vehicles, a long range laser, rockets, other vehicles, etc. If you manage to sneak up on one you can either eject the driver and kill him, kick the driver out and drive off with his vehicle, or punch a sensitive spot on the vehicle in the case of tanks.
Map design is okay, being functional but just that, functional. Some maps are great for large battle and others for "close-quarters" matches. Others are designing disasters. For example, on the map Standoff whichever team controls the vehicles will almost always win, due to the lack of a vehicle busting weapon inside the base where your team spawns. Another map is so large that your are likely to be picked off by a sniper, giving no chance to you, unless you get into a vehicle, which are usually all gone or the enemy keeps destroying them because they are hanging out at your base killing you as you leave.
Next are the gametypes. It is true that straight up (team or not) slayer (kill everyone that is bad) is a good gametype. It has worked for years. However, due to the spawning system, it is a bit broken. In a nutshell the spawning system works like this: you spawn near teammates and away from baddies. This seems good in theory, but it has a prominent flaw. Say that on your team of 4, 2 of your teammates quit/lag out. It seems that you are outnumbered, but this works to your advantage. After you die, you will be spawned far away from enemies giving you ample time to find a powerful weapon, or barring that, a decent ambush spot. The enemy will have to fan out to find you, which of course separates them. If your teammate is at least good he will stick with you. You find an enemy and kill him, because he was separated and looking for you. Rinse and repeat until you win. A general reason to why this works is that you have more targets to shoot at. Say that they do travel in a group. Well your team can spam grenades at them and kill one or two, possibly more. You get 2-4 kills and they only get 2. So unless real tactics are used, something lacking in Halo 3, you are likely to die from grenade spam.
That about wraps it up. Campaign is great, map editor okay if you are into that stuff, multiplayer is unbalanced and broken. You can get good at it if you follow MLG and use the BR and grenades almost exclusively, and I'm sure that if you do get good at it you will have fun, but its a lot like conning people. Until you do get good you will only get bile from people and you will feel bad about yourself because that is what you dedicated your life to at that point in time.
The necessary non-you-just-stink-at-this-game waiver: I got to Colonel rank before I dropped it. So please don't spam me about that.