Things you learned about a Job/role by not playing it

[Atoli]

Not putting regen on the tank pre-pull on healer. It's not impossible to deal with it (even without clicking it off),
but I sure as heck had a really hard time catching all monsters that made a beeline for the healer as soon as they saw me when I played tank.

Not sure if it counts because BLM is actually my main, but..singing songs for my magic DPS on BRD,
because I knew from my BLM that it makes such a noticable difference (even back when all songs would gimp BRD dps).

Not standing close to/on top of BLMs, BRDs and MCHs, especially not on BLM's Ley Lines, when an encounter throws AoE circles at random players soon.
I keep away from them on all other classes, because I know from playing BLM how much it hurts when the Ley Lines get covered by lava because DRG no.3 thought he had to drop his AoE there.

Not healing DRKs who popped Living Dead when I'm on healer. Until I played DRK myself,
I wasn't aware of how much of a waste it is to get healed when you popped it, to the point that you actually never hit 0 HP and then it just runs out unused...>.>

Also, never understood all the MNK "MY STAAAAAACKS" cries and whines and rants until I finally finished MNK a few weeks ago,
after not playing it since 1.0 and therefore being completely oblivious to any stack traits it had.
Didn't exactly change how I play other jobs, but it raised my understanding for MNKs just ramming into bosses and mob groups without waiting for the tank to pull them