Gameplay mechanics aside, Halo 2 was in a different era of the internet. You are probably excited to be playing online and learning lots of things about the game that you'd never thought of from other players. When Halo 2 was new this is how everyone felt. Everyone was introducing new friends to each other constantly, we all wore mics all the time, had hours and hours of custom game lobbies where we'd just sit and talk, show each other jumps we'd found, do the same superbounce (glitch that made you jump very, very high) for a whole evening straight, stupid customs like first to 250 plasma pistol punchout on Ascension/Lockout. There were also a lot more custom games going on - if you had a full friends list or a clan you pretty much had access to a lockout or midship FFA game 24/7. You'd join games that had been going for hours where the top 10 players all had hundreds of kills and you could never hope to catch up, but it was just fun to play.
We played custom games where the rules were fully imposed by the community, and it was up to us to gang up on players who broke them (e.g. Cat and Mouse where 2 players would get in Wraiths and the rest would try to run away in Warthogs, no shooting of the Wraith and no getting out of the Warthog, Zombies where you had to switch teams when you died and had to not use the human weapons as a zombie and not use the sword as a human). Halo 2 had a clan system built in, complete with clan matchmaking games. I used to try out for clans in 1v1s against a player who was in it to prove my worth, and I can still remember winning some that at the time I thought I had no hope of doing so.
A lot of what was so magic about it was that the internet was a different place back then. I remember playing about 5 games of matchmaking with a guy, then we went into a custom lobby on Coagulation (Blood Gulch re-make) and literally just shared our whole life stories with each other over the course of a whole afternoon, and at the time this is just the sort of thing you'd happily do on the internet. We ended up playing together almost daily for 3 years. The attitude in matchmaking was different too, the social lobbies were really social, people weren't worried about KD like they are now, they'd shout more (excitedly) and get angry less. Ranked was super serious, though.
Halo 3 was pretty fun too, the forge and deeper customisation options led to some really amazing gametypes being made that you would just never be able to conceive of yourself. Doing the puzzle maps with friends, the zombie variants, playing matchmaking on custom maps etc was all completely new to me at the time. The Xbox userbase at the time hadn't gone off using mics yet, in the ranked playlists you would still expect all 8 players to be using mics the vast majority of the time.
I guess if you didn't play Halo 2 or 3 online then the idea of the ranking system is pretty new to you, but that was one of the greatest things Halo had to offer, and frankly the game is nowhere near what it was without it. The intensity of the games used to be crazy. If a game was 49-49 then the next point would secure the win and the winning team's ranks would go up while the losing team's would go down. The tension at that point was really massive, and I haven't really felt it since. You really would feel your heart sink when someone died on your team, or guilty if you was you, and inversely such intense joy when your team won and pride when it was your kill. People devised names for every location on the maps and would constantly update you as to the enemy team's location and status in order to give you even the slightest advantage. It's strange to think now because it hasn't happened in so long, but we would be endlessly saying things like "Papa John is one-shot on training with rockets", "They have a guy in shotgun with sniper watching long hall, rockets up in 20 seconds" etc. It was a whole different experience to the social gaming you experience now.
I'll find a way to purchase it anyways... As soon as I find my 3DS and 3DS charger, I'll post my friend code. 
