A grey mob for one player gives the group no XP

Bring a much higher player into a dungeon and nobody gets experience for the kills. Since the hotfixes of 13 November, a monster that is trivial for even one member of the group gives the whole group zero experience. Quests still pay out as usual.

Blizzard put it in one line in the hotfix notes:

Killing 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.

What trivial means

A monster is trivial when its nameplate turns grey for you. It is then so far below your level that it gives no experience at all. The experience guide has the colors. Until now that only counted for the player who saw it grey. Now one grey monster in the eyes of one player costs all five their kill experience.

Trash and bosses

Trash in a dungeon is often a few levels below the bosses, and some dungeons get higher as you go deeper. So the trash at the start can give nothing, while the monsters further in still give experience. The bosses usually pay out the longest.

Kill experience in dungeons is worth more since 8 November, when Blizzard raised it to 60% of Classic Era. A high helper now throws a decent part of that away.

Quests still count

Dungeon quests give most of the experience in a run, and they are not affected. Some groups may still take a higher player along, because a faster run means a faster turn-in of the quests.

In October Blizzard fixed a bug that based every kill on the highest level in the group. Each player now earns experience for their own level again, as long as no one in the group sees the monster grey.