A good PLD uses Flash, Halone combo, and tab targets and Marks his mob (and maybe Scorn for fun).
A good DPS focus target on marks, doesn't attack the mob until tank has threat, doesn't use his heavy hitting skills back to back right after the pull, and only AOE's once the good tank has agro enough on the group.
A good Healer doesn't regen as the tank is pulling or over heal and spam Medica 2.

But you don't always get good players. Ran into a Whm in Amdapor Keep that thought Regen was hot shit and spammed it the entire run, even as the last mob of a group dies he would cast regen on me.... so annoying.

Bad players are a dime a dozen, and most of the time trying to help them is useless because they think they're hot shit.