Its the easiest Halo to kill someone yes, but also the easiest to die in (and you still need 4+1 shots to finish a player). Biggest difference is tempo, wich is waaaaay upped. Noobs don't play it either. So, yeah, this is probably because the game is so casual, soft and sit-back styled. Not. And BTW rank is meaningless. Why even bring it up in a balance discussion?
Not rewarding good players? I have friends with k/ds like 8000/2000 and such. I start a BTO with them, never hear their BRs stop shooting for the whole match (unless ammo is out). Usually we pass four opposing teams until the matches are done because of quitting. Same in Slayer. They're dominating so hard its unseen of. Nothing in Halo3 was ever so slanted really, since they're soo good with Br/Dmr, spawn with them, and these weapons are like lasers in H4. That's how it is.
Btw As far as experience goes, I've played all Halos. Put in 4000 matches in Halo3. You?