I saw this question was originally answered on Reddit, but obviously that's now private + I would like to minimise my engagement with Reddit if possible.
Basically: how does the Mythic World First work? Is it based on amount of successful raids? Is it based on time? Basically, what are the rules, if someone could explain.
Then we would have to say goodbye to any US guild streaming since they would do prog and create strats only for the EU teams to use them and clear faster. Or maybe guilds would do splits while others prog, let the others kill the last boss then clear the tier super fast with established strats.
Nowadays, it's pretty much purely based on when they were released. The current tier includes only the raid Aberrus, the Shadowed Crucible. The previous tier was Vault of the Incarnates.
A "tier" was pretty much defined by a new "tier set" being released. They removed tier sets for a while, but it seems like we're going back to that being a good definition.
Actually "tier" is used literally here as a level or grade within a hierarchy. With new patches typically comes a new higher level raid or higher tier raid. The term "tier set" is derived from that, as it is a collection of gear from a specific tier that provides a bonus when equipped together, not vice versa.
There are plenty of other sets of varying size that provide bonuses, most commonly known ones are pvp sets. But, tier sets are specifically sets that come from a specific raid release.
The only problem with a literal definition is that not all raids are counted. Raids like Mogu'shan Vaults gave lower gear than Heart of Fear/Terrace of Endless Springs, yet it isn't considered its own tier.