It had solid multiplayer that wasn't bloated.
Less than 10 choices makes things more digestible.
I think it's annoying as fuck when games have 50 guns with 100 attachments, but that is the norm today.
I also don't want to be locked into playing a character class like most games today.
Halo 3 was simple AND HAVING 4 PLAYER SPLIT SCREEN ONLINE ON A FUCKING 360 WAS FUCKING AMAZING.
It has a lot of things games don't do anymore.
It's pretty obvious, but the money people at the top don't play games, so we'll never see it again.
It's why Halo 4 wasn't fun. Adding shit doesn't make something better.
Tight. Clean. Polished. Fun.
Especially in a competitive game. People always have "meta" and excuses why they lost. Halo 3 was just "I have quicker reflexes and elbowed you in the face faster", or "i hit you with a sicky". It had very little 'meta' to complain about.
It was very satisfying to play, even when you lost. It didn't feel like you got killed because you're 100 hours behind and haven't unlocked the guns. You got killed and it was fair.