Call of Duty: Modern Warfare was plagued by players running Overkill with M4 and 725 loadouts. Some would add claymores for extra cheese. All of that should be changing soon, as Infinity Ward nerfed all three of them.
M4 is the first in line of nerfs. They aren't particularly harsh but should allow for more loadout variance in the future. Damage range has been lowered and recoil increased so designated marksman rifles might actually have a place in the game, as opposed to being inferior in every aspect to the popular AR platform.
725 is the most hated gun in Call of Duty: Modern Warfare. Players hate it because it has a massive range for a shotgun on top of pinpoint accuracy which essentially made it a sniper rifle that works in the close and mid-range too. This will no longer be the case since the range itself was hit by the nerf bat and spread was increased for both ADS and hip fire.
In case M4 or 725 campers didn't get you, their claymores did if you tried flanking them. These will be easier to bypass as well in the future, despite tricky positioning that campers might utilise. Shooting a claymore will make it do less damage and it will be not lethal if it still hits you. Tripping one in regular fashion will still immediately kill you. On the other hand, the trigger and damage radii have both been decreased so it's easier to navigate around these mines.
There are plenty of weapons that received buffs though, such as UZI which was already great for breaking up entrenched lines of enemies. You can check out the full patch notes on the official Reddit thread .