Legacy loot rules allow older raid bosses to be soloed, making them much easier to farm for cosmetic appearances.
Using normal rules, dungeons from the current and most recent expansion gear drop curated loot based on your group composition - a group with 4 cloth and 1 plate will generally average more than twice as many cloth and caster pieces as plate/melee ones over time. Raids are a little more complicated: LFR loot is still automatically assigned, so you can only get items eligible for your your class/spec, while Normal, Heroic, and Mythic difficulty drops are no longer personal. Instead, they drop randomized gear to allow a broader range of loot that can be freely rolled or traded between players, rather than automatically awarded to specific ones. There's still some debate over whether these drops have any sort of weighting based on the group makeup, but it is possible to get ineligible drops, such as a bow in a group with no Hunters.
Although previous expansions required an 11 level difference before enabling legacy loot, this no longer appears to be the case in Patch 12.0.1, as Dragonflight now falls under legacy loot rules in the Midnight Pre-Patch. This means that all of the dungeons and raids from the past expansion benefit from a healthy damage multiplier, allowing players to much more easily solo them. All bosses also now drop loot from their entire table rather than some being based on the player's specializations, allowing solo or small groups of players to collect a wider variety of gear without changing characters.
That's some pretty good damage!
However, there are still a few quirks to be aware of:
The dragonriding section at the start of Vault of the Incarnates does not benefit from the damage multiplier, though it will automatically end after two minutes regardless.
The Dragonflight legendaries, Nasz'uro, the Unbound Legacy and Fyr'alath the Dreamrender, are still believed to be class-specific. Sarkareth will only drop Cracked Titan Gem for Evokers, and Fyrakk will only drop Fyr'alath the Dreamrender for Death Knights, Paladins, and Warriors (regardless of their active specialization). Interestingly, Rae'shalare, Death's Whisper from Shadowlands can be dropped for all classes, though it isn't a quest item.
Raid skips are not automatically enabled. The quests must be completed normally, which will take several lockouts.
Encounter mechanics still exist. While Normal and Heroic difficulty are largely trivial even at level 80, Awakened Mythic raid bosses still have a lot of health and may live long enough to do something. Their attacks should not typically be deadly, as leech and other damage-based healing is also increased, but some mechanics can ramp up to the point of becoming dangerous and others can just be tedious.
Taking too long to kill Gnarlroot results in having to deal with the Doom Cultivation intermission, which can take a while to solo.
