Allow me
There are 4 difficulties:
Looking For Raid - 25 members, lowest tier reward
Normal Flex - between 10 and 30 members are allowed, bosses/mechanics will scale to the amount of people in the raid
Hard Flex - between 10 and 30 members are allowed, bosses/mechanics will scale to the amount of people in the raid
Mythic - fixed 20 members, cutting edge
I guess the uniqueness here is that a raid is essentially one big ass dungeon with multiple bosses (6~12 depending on which), not a mini dungeon with a boss like FFXIV has. Hence, to practice/do the last boss for example, one has to clear everything again unlike FFXIV where you can just jump straight to it. Also, while you can save your progress, most groups would opt not to because of loot.
For Normal, Hard, Mythic, it's not uncommon for a lot of groups to spend a while clearing it. Even the first and second bosses can make the group struggle for a week (of raiding 2 nights a week for 3 hours). Except in LFR. It seems maybe in FFXIV that people clear shit regardless until they hit a roadblock (i.e. Savage Alex 3). And they hit the roadblock fast.
RE: Gear sets... it's in the game already, but utilised extremely poorly. 2 piece GC (Ul'dahn Soldier's Overcoat) set gives you an extra PLUS THREEE DETERMINATION for example!!! omg!!! lol