What makes Halo 3 amazing for me is the large battles that take place and theres a lot of shit going on at once. Technically the game is awesome, lots of baddies, vehicles, marines, and huge scarabs to take down.
Like for instance in one of the levels where 5 Mongooses with marines as a driver on each of them and on the back is a Marine wielding a rocket launcher. Theres one with no driver but a passanger Marine on the back and he's all yellin "Come on Chief lets rock!". I'm thinking to myself "Hell yeah!". I know something big is about to happen. So I hop on it and haul ass out of the building with the other 4 Mongooses. Once outside I notice lots of things to jump my Mongoose with (which is always fun) then the rest of the area is large and open with a cliff to the right side. In the sky a battle is taking place and straight ahead is a huge Covenant Anti Aricraft Cannon going off. Then a massive Scarab walks over the top of the structure in font of me. Lots of gun fire from the covenant Scarab and all the Marines on the back of the other Mongooses including mine start going off. The Marines are all screaming "Take out it's legs!" Take'em out!". Every Marine then starts aiming for it's legs. Once we imobilized it's legs for a short while the big bastard came down low enough to were when I jumped off the Mongoose and I boarded the rear of it. Then porceded to take out it's gunner and then I ripped the whole thing off it's gun mount to reak havok on the rest of aliens on board. After all of that I found the Scarabs core and destroyed it. The alarms on the thing are going off everywhere, so I jumped off it and luckly enough my Mongoose was still there, but the Marine didn't survive. I got away from as fast as I could, then the beast exploded into pieces.....NIIIICE! Theres a lot of exciting moments in Halo 3 that I will never forget and won't have to because of the replay theater.
It's things like this that make Halo 3 very special. In the theater you watch your whole mission and every level that you've played through. Every battle every gun shot, every single second that you played, you can watch. You can detach the carmea and fly around anywhere on the map. You can put it in slow motion and watch awesome explosions and some pretty badass kills you've done. The attention to detal in Halo 3 towards it's graphics is AMAZING. The replay theater really shows that off because you can zoom in and out of everything in the missions. You can take screenshots of anything and everything, you can take the camera up in the sky and watch battles take place or go right into the thick of the action. Cool addition to the game.
I havent even touch multi-player yet even or The Forge. This game is probably one of the best and complete games ever. Personally it's my favorite FPS ever right now.