Quick definition for set bonus. It's the additional stats that items give that boosts character attributes like intellect and strength. Set bonuses should stack up for each item that a player has, but apparently it's currently not working properly. It seems that Blizzard is really having a hard time with this bug, especially when they asked players for help three weeks ago regarding the matter.

Athas from the WoW Europe forums discovered that the problem was bigger than her problems with the Primal Mooncloth. She noticed that her item set bonuses did not stack with her priest talents (specifically Meditation).

Blizzard GM Eilanai answered that set bonuses do stack up fine with Meditation - the known issue falls on items with identical bonuses. For example, if you have two items that grants 6 mana per 5 seconds (mp5), you might only be benefiting from 6 mp5 instead of 12. Priests were particularly not happy regarding this since the problem seems to be affecting mana regen items.

As of now, there hasn't been any blue announcements regarding the ETA of the fix for this issue. We will keep you posted on further developments.