It's Awesome. Few things really cool, some things, well, suck.
Cool:
Trading weapons with ally's: Say your marine has that sexy sniper rifle you want. You don't have to kill him anymore XD
Duel Wield: Awesome with certain combo's, like plasma Rifle/Pistol and SMG. One takes down shield, the other takes down health.
Covenant Campaign: Usually, I don't like when people make it so you play opposing sides in the same campaign. SW Battlefronts is a perfect example, you fight for one side, then the other. It sucks. But Halo2 makes it go together so cooly that I really did like it.
XBox Live: No more hackers on Gamespy Tunneling

Hi-Jacking things: Nuff Said XD
Campaign: Much longer and stuff. FIve new levels and they're like 5 times as long as Halo1's ones. Although thats bad for the [Subnote 1 (Spoiler)]
And much more I can't think of.
Bad:
Capaign: Ended suckly. Just ended in the middle of something important. Damn

XBox Live: THey have no optimatch. YOu can't get a list of servers and choose one, it chooses one for you no matter what.
Uhh?:
One question. Ok, the Master CHief can flip a 66 ton Scorpian Tank, right? Well, why can't he duel wield rifles? Just a question. Like I know it'd be hard to reload and the recoil would suck, but hes uberly strong, I think he could use two. Just imagine two sniper-rifles *drools*
Oh Well.
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Spoiler:
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1) Although the longer levels will make you wanna cry when you get to the library. Just take the library from the other one, make it three times as long spanning over two levels. *Shivers.
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