PLDs have it the toughest definitely and I agree about the mistake SE made by giving PLD their tank stance 10 levels later than WAR and DRK. As soon as you can wear that soul stone, you should have your tank stance and level 30 isn't too bad at all. That's 15 levels that you have to endure the flash & overpower spam and marking your targets (and praying DPS follow your marks). It really does force you to watch the aggro list of mobs so you know which one you need to give that extra enmity combo to/lob/tomahawk while weaving in flash. It really is EXTREMELY easy to lose hate at those low levels, even to a single AoE heal; but I still think what you learn from that outweighs the negatives. For tanks that are first timers (and even more so, first time tanks on their first MMO like me) it is truly disheartening and can make you want to give up. Leveling WAR was pretty easy, but PLD made me want to give up altogether so I understand that completely. But, I think I gained a lot from that experience and I'm glad I didn't have a tank stance until level 30 WAR (still HATE that I didn't get a tank stance with PLD until 10 levels later)
As HW was introduced a lot of people in my old FC wanted to try tanking for the first time and went with DRK. They were instantly at level 30 with tank stance and had no idea how to hold aggro even in HQ BiS crafted gear. A few pointers and forcing them to level PLD and WAR w/o their tank stance to get their cross-class skills really did help them out a ton and they became much better. I really do think that them having to level PLD and WAR really gave them the experience they needed to understand how aggro is generated (not just from themselves, but by other party members' skills and attacks) and how to overcome deficiencies introduced by gear as well as the missing tank stance.
For trials, I never really had an issue with damage mitigation; positioning, aggro generation etc. I learned these through dungeon runs and dungeon bosses. Specific positioning for specific trials did take some learning; but it was generally, hold at A; move to B when X happens. What did surprise me were the hard and extreme primals, this is where I learned the tank swap and tank buster mechanics which is something that I'd never had to even think about. It wasn't hard once I understood it, but those were the only real surprises.