Niobe man... dood, you just got pwned.
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I've played Halo 2, I have beaten it, and I can say that it wasn't great. It was an average sequel, but ntohing ground breaking. Wow, you can hold two weapons at once (is this really THAT innovating?) the graphics are good, but so what? The gameplay had nothing.
Melee was decent, but nothing to jizz over. The actual game was pretty easy, and it's ending left me with an emptiness, a little void that could have been filled with a solid ending. Halo 3 by the way hasn't been confirmed, infact last I heard, it was not planned. If someone has an article proving otherwise, go for it.
Now, Half-Life 2 has a groundbreaking new engine (the physics, and character expressions are top notch. Best out their). The AI is apperantly very sexy, and cool. The ability to mod it makes it THAT much better, plus the length of the game, and the story is very intruiging.
Think of how much fun you had with HL's multiplayer stuff, now imagine that on a new engine, full with real physics, and crazy guns. It'll blow your load.
Just look at the poll though, it shows you how much people are expecting out of HL2. It isn't even out yet, and people like it better then Halo 2, hahaha.