Blizzard recently released a large set of hotfixes to dungeons, quests, NPCs, items, and more. These hotfixes had a change to dungeon experience, where if a mob is trivial to anyone in the group it will grant zero experience. Mobs turn grey when a player is too high-level and the mob is considered "trivial".
While dungeon experience is nerfed in WoW: Forever at 60% of the original values, all experience still helps you level up. Inviting a high-level player to a dungeon can now remove a decent chunk of raw experience points from mob kills across an entire dungeon.
BlizzardKilling enemies in dungeons now awards zero experience to all party members if the level of the enemy is trivial to the level of one or more of the party members.
This can get complicated, as trash mobs are typically a couple of levels lower than bosses and many players want to be the appropriate level for them. Some dungeons have the level of trash increase as you progress, so there's potential for the trash to give no experience at the start, but will give experience later on. On the flip side, with quests giving the majority of experience in a dungeon, some players may be okay with inviting higher-level players still, as even if the mobs give no experience, they can complete the dungeon faster and turn in their quests faster.
