Halo 2's way too testy a subject. They did a lot right. They tweaked the gameplay, and added some seriously major kick ass shit. Hijacking, dual fucking weilding? Bad ass to the max.
But, the MP balance sucked. Worse than HL1 pistol whoring. Everything's sniping, or super up close. There's no good mid-game, made worse by, well, super strong sniping and melee. Plus, with all the graphical hooplah, they sacrificed some of the great wide open spaces of one, with corridors, smaller spaces, and less detailed (and not as well layed out) larger areas. And, not that it's the game's fault, but the online community fucking blows. Plus, the actual missions weren't great, were (as Vyse said) been there-done that, the Brutes were gay, the ending made your jaw drop (in the worst way).
But, there's a solid base for H3, and lots of the problems can be easily fixed. If I don't get a huge fucking Earth rumble, with massive damage and world altering widespread destruction and massive fights, I will be pissed. Goliath also rules. C'mon, it's Goliath.