I'm starting to burn out on this one a little. I'll probably let it rest a day or two. Edit what I have for consistency and coherence then come back and add what's neeeded. Man I need to start using plans.
Now where Halo shines, is of course, in the multiplayer and here's a stickler, any review that omits multiplayer as a factor is kind of missing the point. It'd be like reviewing quake 3 arena on the strength of it's singleplayer campaign (piss poor) and a lot of no mainstream reviewers seem to be getting away with this. So let's get it straight. Halo 3 is allright if it's just you, or you and your friends saving the universe but you're only getting a fraction of the big picture. Sure, rent the game and have a run through the singleplayer, it'll burn a few hours and you'll have a few laughs but to really get the most out of the product and to see why reviewers are dropping those (inflated, slightly) big scores you'll have to invest in Xbox live.
Gameplay wise, a lot of the strengths of the single player game still shine through. The game is robust and quick, with a fairly forgiving difficulty curve in most game modes. Items that were worthless in single player are still fairly worthless in multiplayer. You won't, for example, suddenly find a new use for the flare but you will see a bubble shield deployed every now and again. The servers are solid, I rarely had any disconnection problems that weren't the result of the whole live network itself going down.
Now to be fair. There isn't a whole lot of depth here and you'll very rarely have to make any kind of tactical decision beyond 'shot gun or assault rifle' and most of the gamemodes neither require teamwork nor encourage it. You will occasionally find an organized team that move as a unit and blow the shit out of you but they are a rare breed. Most of the gamemodes simply boil down to random groups of people shooting at each other.
The game does still have any annoying element of rock paper scissors to it. Regardless of your skill level, if you're caught in a situation with the wrong weapon you'll often find yourself dead incredibly quickly. Some weapons also have a hefty learning curve while others are essentially instant murder machines. The rocket launcher murders everything in it's blast radius which is about the same size as your average nuclear explosion and the shotgun usually kills instantly if you're unlucky enough to get caught up close. On the other hand the battle rifle, which dominates in middle ranges takes a little bit of practice to use effectively as does the carbine, sniper rifle and the grenades. But they do reward practice. At middle ranges a decent assault rifle user can take on two or three enemies at once and come out on top. On the other hand a lucky shotgun user can sometimes decimate your entire team if he gets close enough. The shotgun is the games equivalent of the Blue shell. The great equalizer.
So the multiplayer game generally has it's shit together. It's accessible but rewards practice and your time spent in the game translates into ranks and experience points so you can pretend you're actually doing something with your life but on the whole I wouldn't recommend the game for long term players.
It's awesome in short bursts and it's always going to be one of those games I pick up every couple of weeks or so and spend a few hours getting shot, but the game lacks any depth beyond the combat.
Once you've got the basics down you can hold your own in any game mode without ever changing how you play. Each mode (with a few notable exceptions) essentially just play out as deathmatches with bonus objectives. This is a bit less notable at the higher levels of ranked play but you'll spend most of the games in a variety of wild west shootouts.
Halo 3 does provide a lot of options and opportunities for new experiences but the game browers does it's best to stop you from enjoying new gametypes or modified maps. One of the best examples of this is the game mode infection. Infection is, in my opinion, the best game mode in the game. The game starts with every player save one equipped with a couple of decent weapons, usually a shotgun/pistol combo and the remaining player is the 'infected' and given a plasma sword and quite often the ability to run very fast and jump very high. Each time the infected player kills a regular player that player becomes infected zombie style. This leads to some totally awesome situations. I've seen a variety of defensive strategies ranging from barricading themselves into a building on sandtrap to creating a convoy of vehicles that cover each other as the infected approach but eventually the infected will win. One of my fondest memories of the game, is during a particularly spirited defense of an elephant on sandtrap the infect began to overwhelm the regular players. Sensing that staying on the sinking ship was perhaps not the best course of action me and another player appropriated the ATV inside the elephant and took off out of the back just as the elephant crashed into a ditch causing an awesome dukes of hazard style leap over the approaching infected and a scream of "FUCK YEAH" to escape my lungs at about three in the morning.
Sounds great eh? You want to play that? Well tough shit. You can't.