The hardest thing about tanking for me is finding a time to practice tanking. I've found that more then all the other roles, tanking requires the most in-game practice time. Stuff like what HP threshold do you need CDs, how high can you let everyone else's aggro get and still be holding it yourself, enemy placement, etc. can't really be learned by watching a guide. Tanks have to learn that by doing it (and often failing at doing it) so they can get a feel for how their role works. And so that the next time, they'll know what to do.

The only problem is, most other people in the game aren't willing to give newbie tanks the time to be well... newbie tanks. Try to go slow because you've never tanked a dungeon before (or the last time you tanke it you screwed up)? Everyone else wants to go faster even though you know you can't take three pulls at once. Try to learn CD management on old trials you know the healer can heal you through so long as you don't mess up mechanics? They start complaining about not being able to DPS as much as they would like.

Granted, I have had runs where the other party members are willing to let me go at my own pace and figure stuff out and those are the runs I actually learn things in. But most of time I get runs where party members think I'm going too slow and pull extra mobs on purpose. Even though I know I have no CDs left and what they just did will result in a wipe. As for Trials... lol, no. Unless you're tanking when they first came out, you're better off avoiding those as a tank until the next expansion. Then at least you won't have to worry about dying as much if you're CD usage is wrong. You've probably learned how it's supposed to be positioned already from when you cleared it as a DPS/Healer at least.