I never played the multiplayer, since I was 9 at the time, but the campaign was honestly just the best thing ever. Except for Halo 3 IMO, but honestly all the glitches you could do, the vast environment, the submerging of every level, and the great story line. It was really just such a great game.
Halo 2 was the first introduction to real Halo multiplayer, (aside from Halo 1 LAN parities and PC link ups ... which btw there are some awesome vids of physics exploits haha)
It was also really awesome to have a sequel, finally, looking forward to playing as MC again is an anticipatory experience I dont think will ever be matched.
The way Halo 2 ended, you knew there was going to be a sequel, and without spoilers, I remember playing through the entire thing with a few friends, and right as the last cutscene blacked out, I made the joke "Wouldn't that be the ****iest thing ever if it just ended and was all like 'see Master Chief return in Halo 3'" and then it really did end and we were so pissed off lol.
The game had its flaws, but nostalgia is like a gold plating, and the time that passes is a polish that makes it shinier and shinier in your memory.
In online play, it seemed that people were just more social and having more fun, it wasn't as much about ranking up as it was about just getting online and playing.
In my opinion, it is more about the FEELING of Halo 2 that, in retrospect, makes it the fondest memory of the Halo series, because as an over-all experience, it was more fun than I have ever had in a video game.
Players are looking for that experience to be repeated, but its kind of like watching The Matrix for a second time... you will never again have that "OMG" revelation moment because you already know what happens.
DEFEND YOUR 7-11 ! ! ! ! ! C'mon, Spartans, those Doritos aren't going to eat themselves ! We need an Or-Dew-nance drop on crimson's position NOW !