Though not as in-depth as the original, Halo 2 still is a remarkable expirience.
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This is where the game truly shines. The graphics have vastly been inproved since the original game. The models are alot more clear and precise. Textures look more realistic and the animations (including the lip syncing!) have been improved as well. The only drawback on them is that they are sometimes too much for the Xbox's now dated hardware, which makes some textures take longer to load (mostly in cinematics). Sounds/Music
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The sounds in Halo 2 are great. Each gun has it's own distinctive sound, and some have been changed, most notably, the Plasma Rifle. The Marines and Covenant also have more speech, which is really saying something, since the first game already had a ton. The enviroment sounds are also great. There's nothing like creeping through an ancient installation and listening to all the different machines. Now on to the music.
The music in Halo 2 isn't as good as Halo's, but is still some of the best. We now have bands such as Breaking Benjamin and Incubus playing Halo themed music for us in game (though the singing was taken out, thank God). It is just as, if not more diverse than the original Halo soundtrack. Gameplay
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Halo 2's gameplay was somewhat disappointing. It seemed to be more multiplayer based than singleplayer. Although it is still fun, it's just not the same. Now, on to the good news. There are now more Marines to fight at your side, and some new characters to learn about (some of which being Covenant).
Story line
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Earth, the gem of the UNSC Empire, is under attack. In Halo 2, you are flung into action when the Covenant decide to invade Earth. Now the Cheif will stop at nothing to end the Covenant invasion. Again, I don't want to ruin it for you.