The game was faster and it seemed to be more competitive for some reason. I also think that it was the fact the people could play ONLINE at the same time with kids from around the country/world. I remember when I first played online when I was in like 8th grade, my mind was blow away. The maps were the best out of any Halo in my opinion, for example LOCKOUT, beaver creek, warlock, headlong, zanzibar, etc. etc. the cqc actually required skill to win (no mutual beatdown crap). If you were good at bxr/bxb, then you ACTUALLY stood a chance if you had less health than the other guy. "What? no way?!?! I've never heard of such madness!" Yeah. Also the BR, it was the weapon of Halo, 'nuff said. 4 shots, double shots, quad shots...ahhhh the best.
Aside from all the matchmaking, competitive ranking, etc. I LOVED the aspect of glitching/superjumps. It was some of the most fun I've ever had playing a videogame. Trying to get out of maps, finding new places outside of headlong, doing that new superjump some random kid found, tank flips on coag...I could go ON and ON...
Simply put, this game provided me with the most fun I've ever had while playing a videogame. I played from the first day it came out until its end a few years back (still a mistake that they did that). Halo 2 will be my all time fav game online for sure.