Beyond these human elements, encounters generally boil down to math, with some rng thrown in based on who gets hit by what unavoidable thingamabobber. The encounters stay open and available for anyone who'd entertain the offer, which is why they aren't exclusive.
Attaining world first on a given tier is an exclusive game device. Because no matter what, only 10/25 people are going to get it, and once they do, it's impossible to achieve again until next tier.
Any item that is considered "one per server" (I know other games have had such things, I don't believe WoW has anything like this?) is exclusive content.