This is not intended to sound like I'm pissed off at Bungie or anything, but when you first see that the classic Halo is on the marketplace on the 360, it makes you wonder.
What was added to the game?
And the answer?
Not Xbox Live.
Seriously? I know Halo is fine the way it is. But could you imagine playing over Xbox Live with it, the unchanged Halo: Combat Evolved in all its glory? Tell me with a straight face that you wouldn't enjoy CTF or Team Slayer or Race online with other people. Or maybe even online co-op.
I don't think it would have killed anyone to add this in. That is the one thing I never understood: why Bungie never added Xbox Live as a feature for Halo 1.