People seem to have trouble with whats an exploit and what isn't. I know the line between "Clever use of game mechanics" and "exploit" can be rather blurry, however I think there is one clear thing about what an exploit is:
Purposefully using an unintended bug to bypass or simplify a fight mechanic.
A few examples:
Ensidia used saronite bombs to exploit their 25LK kill, instead of reporting the bug and doing the fight normally.
(I forget which guild did this) bugged Yogg+0 by having a paladin healer evade bug guardians in phase 3.
Both examples are a case where something unintended caused the fight to become trivial.
I never got why people think things like Class stacking and "gimmicks" are exploits.
Paragon's H25 Nef kill baffled me when people said they exploited because they stacked druids. This kind of logic simply does not make sense to me. The recent thread that popped up made me think about this, but it's not the first time I've seen people claiming such things are exploits. Are people seriously just that jealous?