There was no bug with zolo, it was just poorly balanced with specific conditions.

That rogue talent that lets them live and take 90% less damage for a few seconds got nerfed. It is still possible with the right gear to obtain nearly the same effect it had before. It must be an exploit.

Shaman's lightning bolt is getting a lower co-efficient in wotlk. Using lightning bolt now is ok, and using it after is ok, but stacking spell power to get the damage back up will be an exploit.

People were clearing the level 60 5-mans too easily with 10 people so they lowered the max number of people allowed to 5. Anyone clearing these instances quickly now must be exploiting.

People were getting 100-200g/hour by killing Jania Proudmore pre-bc. These people even got 3-day suspensions apparently. Anyone getting near this amount of gold per hour is exploiting.

And if the only "bug" here is the exp/hour, then looking for zolo-like encounters should be fine as long as they don't give too much exp, and how much exp is too much exp is for blizzard to decide when regarding "exploiting".