The day Halo 2 came out, I believe that Xbox Live changed forever. I don't even know, but I'm guessing ~80+% of Xbox Live Subscriptions the next few years were solely from the release of Halo 2. Much of what was put in, is standard in today's shooters.
Matchmaking, matching players based off of skill level, was born then. I wanted to see if anyone agrees that Halo 2 revolutionized Xbox Live, for what it is today.