I've played just a little, I'm in the first mission right now. I don't know if I want to play too much, since I still need to play through Halo 3 and don't know if that would spoil things or experience them in an order that doesn't make sense. (I'd appreciate if anyone can comment on wether that would be the case or not)
The overall mood and tone seems different, in that you're supposed to be a weaker unit with little or no support, which is nice. I really like the pistol, and the new silenced SMG. Unfortunately like all other Halo games, you run out of ammo and are forced to swap out for alien guns, which are mostly lamer, and in a lot of cases inferior. Or maybe they're not inferior but I just never gave them the chance becasue they "feel" lame compared to the human weapons.
I was fighting a brute and he got in a little close and melee'd me, he was lunging and I was dodging. When he died, I realized my character was breathing heavy, which sort of emphasized that I'm not a spartan anymore.