You don't shoot humans in Halo
In multiplayer you do.
JFG as well. In fact, you also shoot dogs 
But is there any blood? That's the big factor usually.
Before E10 rating was created, there was some ridiculousness, like F-zero getting rated T. >.<
lol what? i dont think i ever noticed the rating for that.. what kind of description did they use to qualify it as teen??
Ah yes, F-Zero GX. The ratings descriptor mentions three things:
Comic Mischief
Mild Violence
Suggestive Themes
My guess would be that it is due to the suggestive themes, whatever those are. Does Captain Falcon hit on the ladies or something?
I still say Super Smash Brothers Melee would have gotten the E10 rating if it existed back then. I know that SSBBrawl received a T rating also but it is more gritty, in part due to Snake. Though I still don't think it deserves a T rating.