about the loot,

is it just random, i mean totally random without any rules? you kill the boss one player open the chest and here you go and item? that sometimes is not related to any of the current party classes or needs?
although nothing go to waste there always some grabs it specially if it was green, but playing through one dungeon many times to get one particular item or trying to complete a set is so annoying, like i was playing my bard i just open bryflox i wanted that infantry set it looked cool i played it many times i manged only to grab 3 piece of Armour and my level became like 42 and i no longer need the level 32 set, or i'm playing with PLD from lvl 30 to 38 just to get the shield at haukke manor but never spawned once.
my point is, i think we need better loot disruption system.
I think the number of times you run the dungeon should count to the loot system or there should be separated reward system for dungeons that help you to acqire the desired loot.

When I got into level 32 with my tank classes I had full Infantry and full (what's the tank set there) from Bray. That since I had ran already 5 classes through it. You have 12 jobs to level through Bray, it just takes some tenacity.

The loot is RNG generated based on a loot table. The only rule is when a specific bit of gear drops from a certain boss and not others. Unfortunately RNG is RNG, chances are you'll get all the sets while leveling other classes, and as you found out, its not worth farming a dungeon for gear after more than a few levels because you wont need it. On the bright side you have 3 pieces of the set for your next DoW class you level :)