"It's overrated!11!"
Why do people hate games because others love it?
No one will ever know.
I myself rarely like to play first person shooters. However, Halo 3 seemed different from the rest of them.
It had nice visual effects, it had COLOR. (not another clone of a modern 'realistic' game where everything is brown and grey.)
It also had unique weapons, not a whole group of machine guns that kill in 2-3 hits.
The bugs and physics were fun to play around in.
Online was just plain fun. (I don't play ranked, I could give half a stuff about numbers.)
Beating co-op with my bro was fun while it lasted. (On normal, and no idc about achievements.)
But once I played it for quite awhile, like every other fps (except goldeneye for N64) it got boring.
I do say it lasted longer than the others. (except goldeneye, best fps ever)