Halo 3 adds many new things to the franchise:

Forge mode (kinda like G-mod...allows player to edit the things on the map as well as add things to it....while people are playing in it),

4 player co-op,

Equipment (i.e. Portable Grav Lift, Bubble Shield, Portable Turret, Energy Drainer, Trip Mines,Flare, etc...),

Meta Games (now the game scores you on how you play...adding to the replayability and creating some achievement opportunities :)),

Saved Films (allows you to save Singleplayer and Multiplayer footage, change camera angles, focus on different people, record, take screenshots, play it in slow motion, fast forward, send it to friends, upload on sites,etc),

Ability to create your own powerups in multiplayer,

A changed control scheme that feels much better yet still Halo-like

Everything is now destructable (Like Dropships and Scarabs)

Rip out turrets and move in third person(not a HUGE thing, but still adds some strategy and change in gameplay)

Tons of new vehicles (Hornet, Prowler, Elephant, Brute Chopper, Mongoose, etc...)

New weapons (Spartan Laser, Gravity hammer, Flamethrower, Mauler, Brute Spiker, Spike Grenades, Flame Grenades, and modifications to old weapons like the new and improved Assault Rifle, the more powerful and un-duel-wieldable Needler, etc)

Many new Multiplayer Gamemodes (i.e infestation, Mosphit, VIP, etc)...not to mention that you could create your own)

AI is vastly improved (Brute Packs, formations, enemy reacts to different situations accordingly and most importantly, are much more aggresive and smart overall)

New and improved enemies

Over 40,000 combat Dialogue ALONE that immerses player into the experience and setting

The end to a truly epic storyline, as well as answers to mysteries such as the Flood, Forerunners, the mysterious connection between humans and forerunner, etc..

Graphics are improved and look beautiful (HDR, crisp and clean textures, Real time reflections off anything metallic as well as Chief's visor, Realistic Physics, Realistic Water effects and physics, Nice smooth framerate that looks and feels over 30FPS, realistic vegetation, etc)

Achievements :)

And many more things we'll see when the game comes out.

Not to mention the game plays like the other halos, so the superb gameplay elements from the past are still there.

I don't want a whole change in the Halo series. Only additions. So I don't need innovation when it comes to Halo. Only evolution.