I played with spas12 for a while... as soon as you have range or damage on it, it's to strong in my opinion... i had some triple kills with it. Acr is no skill needed, you got a point there.. i dont play with AR, that's for players which suck at aiming. mp7 as well.
I mostly play with p90, pp90, msr , l118
Spas-12 in MW2 did 40-20 damage as a secondary weapon whilst in MW3 it currently does 30-14 damage as a primary weapon.
The Spas-12 is not overpowered in MW3 and using it is more difficult than using a P90 & PP90.
In my quest for all gold guns, my last two to level up are the KSG and Spas. I have heard comments about my use of all of them, with the least hate coming in for the Model 1887. I think that is because most people have tried the Model out and find it hard to hate someone who handicaps themselves with that POS. I say that out of hate and love for that gun.
My most recent gold shottie, the AA12 has started to rack up a lot of haters. Once you have range and extended mags on it, you can wreck teams on maps like Seatown. Nothing brings out the hate like the "no-skill" auto shotgun. I just thank them and encourage them to try them for themselves to see how they do. Until you get range on it you can't kill much past knife lunge range, afterwards you are good for most rooms and short hallways.
To answer the OP, people hate the OHK (as has been pointed out), I know when I rage, it's when I die without a chance to fight back. If it is against Snipers and Shotguns, you know that unless the opponent is very good, you can usually counter with superior weapons. If it is MK14s/Type 95s, you need to pull on your try-hard underwear.