Halo 3 is all you would expect from Bungie and perhaps a little bit more. It should make you say 'Bioshock who'?
The gameplay is almost totally unchanged, except for the addition of equipment and weapon balancing, and the addition of some new stuff, such as the flamethrower (awesome looking but useless in MP) and the Spartan Laser (hardest achievement in the game is tied to it) and other things like that. I agree though, if it ain't broke, don't break it.
The single player campaign picks up a pace or two after the cliffhanger ending of Halo 2, with you flying towards Earth and into another battle versus the, more Brute based Covenant. The games story and plot stick to the normal Halo style, nothing too crazy (though a few good characters die, not saying who, but you know...). Very solid campaign, even if Legendary seems a little easier than before. (I can beat the game on Legendary, as apposed to Halo 1 or 2 where I couldn't). But even there, skulls add on challanges like having no reticle to aim with or enemies being just plain tougher. And now you can actually save a skull as apposed to having to find it over and over again ala Halo 2.
Multiplayer is awesome, maybe even as good as the Halo 1 days (no sniper pistol though, sorry fanboys). The maps are solid (Isolation sucks IMO), but with the autoupdates and DLable map packs, you're bound to find something to like in terms of maps. (Lockout and Sidewinder are being remade for Halo 3 soon, more to come).
Speaking of maps, you can make your own! Or rather, you can add stuff to existing map geometry. Meaning you can add weapons, equipment, crates, fusion coils, grenades, Warthogs, and pretty much everything else to a map and create a pit of doom that is either a really fun killfest or just a piece of crap. Along with improved custom gametype options like gravity, speed, color, damage resistance, etc., you can make anything just about.
And the Theater just wraps it all up. You can make movies of epic battles fought over a cadaver-covered landscape or just an awesome no scope you scored on your friend and a minute of you corpse-humping him.
With all of that there is surely something you will like, I like all of it, explaining why I thought the game deserved a great score.