As for what increased exp wise I think it was just the bonus experience for queuing randoms as that is all I noticed increase after they slashed exp to 1/10 (10%) when a character is in the group that has outleveled the current content

Dungeons get tricky exp wise if not using the random queue...basically if anyone in the group...when entering a dungeon by foot is outside the new level ranges...the penalty kicks in...

so 15-60, 61-79, 80-89, 90-100 groupings are now subject to penalty exp gain....even if the mobs are still green to say a 65 and everyone else is 59 on the 15-60 dungeons...everyone gets penalized

Exp tapers off quickly too each level over max even if the group is all the same level...so max exp may be at lvl 60 and by 61 -65 it drops by 20% each level(not sure exact % offhand but it was quickly not worth the time)

Dungeons were probably too easy before but ahead of forcing everyone to level grind for Heritage Transmog on the new races and the Vanilla servers planned...they went overboard making stuff harder

98-110 goes much faster in legion just doing the 6 Invasions quests every so often grouped with a 110

90-98 went decently in draenor doing rares and chests/treasures as opposed to dungeons on Frostfire and Gorgrond

Some old dungeons feel tougher than legion Mythic now at the correct level (110 mythic + still tougher due to some affixes and bugs) ... even in all heirlooms

Ghostlands still are fastest exp questing in 1-60 but last for only 5-10 levels worth of exp