Knowing how to gain aggro quickly
I leveled my PLD after maining MNK for a while, and now it's my main. Like everyone says, Lv 30 ~ 40 were a nightmare. Heck, with a trigger Happy Ninja and Black Mage, you'll likely lose aggro even at iLv 100.
What I've learned is that at Lv50, attack cooldowns are the key to holding aggro.
As a PLD you have several aggro gaining actions, but note that most of them share cooldown time: the halone combo, flash and shield lob. You can't do them one after another.
On the other hand, Circle of Scorn and Spirits Within have each their separate cooldowns, so you can do them right before or after any of the actions noted above.
So for the initial pull I normally use
Shield lob > ( cooldown while enemies get in range ) > Flash > Circle of Scorn > Spirits Within on my "main" target > Flash ( cooldown ends while you do Circle of Scorn & Spirits Within )
This normally buys me enough time to see what the DPS are doing and react accordingly.