Is it true that holding aggro 100% of the time isn't possible at low levels?

[Pixelmancer]

It's both true and irrelevant. Low level dungeons are so easy a Cure I takes a tank from 1 HP to full. Speaking as a healer, I don't really care who has aggro in low level content. Until the middle to late twenties it literally makes no difference whatsoever to me if the thaumaturge tanks.

[RiisWolf]

It's VERY possible to not keep threat 100% of the time as a new tank with less than high quality gear thanks to these factors.

-Low Threat Multipliers.
-Lower attack power than DPS.
-First attack in combo does NOT have a threat multiplier. Yes, this creates a problem, especially when you're dealing with a black mage.
-Patches mostly buff stances, which you don't have until 30 or in Paladin's case, 40.
-Patches upgraded level sync to lower gear to equal to HIGH QUALITY gear of that level, which means it does nothing for newer players in normal quality gear but make it harder to hold threat anyone and everyone with better gear has been buffed.
-Gear impacts performance more than DPS and Healers. No hate=useless tank. Must keep gear up to date or you're guaranteed to lose hate no matter what you try to do. Don't let a lack of skill in DPS fool you.
-Many don't know popping buffs also creates hate, although a very small amount. So pop some buffs like foresight in the middle of combat, NOT before.
-The ranged attack all tanks get at 15 is a great way to start pulls from a far, but the threat it generate is weaker than most, if not any other threat attack. A lot of tanks don't know that.
-Provoke must be followed up with some threat or it will fail.
-DPS don't have to be that much stronger than you to overpower your threat while out of tank stance.
-Sword Oath
-Black Mages can deal big damage right away, unlike most others even at the earliest levels.
-As Marauder: Threat always comes first but try to use maim. Flash is great to have. Overpower is your best friend.
-As Gladiator: Threat ALWAYS comes first. If you're fine with less damage for more attention and/or defense, then you're fine, if not, then get off Gladiator.
-As Dark Knight: Do Marauder and Gladiator first.
-There is nothing advance about keeping hate. It just has problems. Don't make it harder, just get threat on them all FAST and reduce the damage you take.

[technole]

To be honest; I kind of like having to actively battle to hold aggro. Not many MMOs these days actually push me to stay on my toes like that.

You'll get to put that in-practice plenty at end-game level too, because you'll run into good players that know their rotations well, even will potion up buffs in their openers and what not. It will make you, the tank, play on-par otherwise they will rip aggro off.

My favorites are the slack tanks out there that think they can pull whatever they want in DPS stance, without a shade walker buff, then wet their pants realizing they don't have slack dps present.

Being able tank/dps stance dance against it, while keeping threat can be quite a thrill.

[Neophyte]

I'd suggest_

• If you don't have your job yet, get Raging Strikes (BRD. LVL 4) and maybe Internal Release (PLG, LVL. 12), as a WAR you can get Invigorate (LNC, LVL. 22) or Flash (GLD, LVL.8) to help managing your TP if you over-do it with the Overpowers.
• Meld your gear! The stats on low level gear are extremly low and strength materia I or II are very, very cheap. If you have a ring with +1 VIT you might meld a VIT I (or vice versa, since there are only STR acc <lvl. 25 I think?). It doubles your AP from those accesoires.
  Melding ACC also helps beating up higher hunting log mobs, that's what I do on the left side usually to be able to do leves/fates/mobs more than 5 level above my leveling class.
• Keep at least your weapon up to date. Since Off-Stats and mainstats only rise by 1-2 per gear upgrade anyway and Weapon Damage usually scales with 8-9 mainstat, always make sure to have the best weapon possible. An weapon upgrade sometimes gets you +1-3 weapon damage, which equals a full upgrade of all your other gear.
• If you really have to fight with a DPS to get hate back, just don't do it. Don't waste 2-3 hate combos on a mob you won't get anyway. He will finish the mob off, while you can secure the other ones.