Abuse might be the wrong word, it's just that things that seems small in pve tend to have huge impacts in pvp. The thorn dot doesn't matter much in pve, but since it gives wall hacks and delays health regen in pvp it was a huge deal to have on a gun. Turning a fusion rifle into an auto rifle with the vex was cool, but since it could crit it's time to kill in pvp was ridiculously low leading to it's large string of nerfs. Basically anything that creates an advantage throughout a gunfight will have a large impact in pvp. The mida being another example. The exotic perk while it doesn't look like it makes much of a difference, if you slap it on an already solid weapon turns it into a force that runs the crucible.