Lancer tanking in TERA is the best implementation of tanking in any MMO that I have played.
Now you should take that with a grain of salt, as my Lancer is only level 28, and that is far from endgame tanking. But I thought I'd take a look at the mechanics of tanking in TERA, and what makes it different from WoW-style MMOs.
The Lancer is a shield tank. Her signature skill is Stand Fast. When you press the Stand Fast button, the Lancer brings up her shield to block enemy attacks from the front. She braces in place and continues blocking for as long as the button is held down. Blocking the enemy's attack is guaranteed, and absorbs a fixed amount of damage--not a percentage--from every attack. As well, players behind the tank are also shielded from damage.
One thing I should add here is that blocking an attack looks and sounds awesome. The character gets rocked back a bit, and there is a very distinctive clang as the attack connects with the shield. If the monster does an charge attack, the tank gets pushed back while continuously blocking, and that just looks spectacular. Bluehole did a spectacular job of making tanking appealing on a purely aesthetic level.
The key element here is that the amount of damage blocked is extremely large. Almost all attacks, including boss specials, are fully absorbed. Essentially, while the Lancer Stands Fast, she takes zero damage.
However, while she is blocking, the Lancer cannot deal damage or gain threat or mana. She must drop the block, making her vulnerable, in order to attack the enemy. And threat matters. The Lancer needs to fight for threat to keep the boss from going after the DPS or the healer.
Those two mechanics produce a marvellous tension. The Lancer must block, or she will take too much damage and die. The Lancer must attack, or she will lose the boss's attention. As well, the boss telegraphs his attacks through his animations. So the Lancer can observe the boss and identify times when it is safe to drop the block and attack, and the times where she absolutely must block.
I think the key here is the absoluteness of defence. In WoW, the choice between threat and survivability always went to survivability, because every inch made the tank less likely to die, and lessened the burden on the healer. In TERA, you know that if you block correctly, your defence is absolute, and so you are free to spend resources on threat. It moves the threat/balance trade-off from gearing to gameplay.
For example, right now I'm spending all my specialization points on threat. This means that fewer attacks build the same amount of threat, and thus I can spend more time blocking, and have more room for error.
The last part is positioning. WoW tanks spend a lot of time and effort positioning and moving bosses correctly. In TERA positioning doesn't seem to matter as much, as the bosses seem to run around like monkeys anyways, and the other characters are agile enough to dodge out of the way.
Now, there are downsides to the TERA model. In particular, it can be very unforgiving. The first time I tanked an instance, I dropped my shield at exactly the wrong moment, and got nailed by a boss special, which killed me because I was already a bit damaged. Tanks are somewhat rare in TERA, so maybe this is a cause. (Though it might be just because there are so many DPS classes compared to the tank classes.)
Perhaps the secret to the success of TERA tanking design is that the tank has control over both threat and survivability during gameplay, and has to balance both. In vanilla WoW, the tank didn't really control her damage intake outside of long-term cooldowns. Instead that was a function of her gear. She spent all her resources on threat and positioning. In modern WoW, threat no longer matters. Instead, the tank will spend her resources on mitigation and positioning. Neither style has the enjoyable tension that TERA tanking does.
Or perhaps the secret is just making the tank more responsible for her health, and the healer less responsible. You still need a healer in TERA, as it makes life a lot easier. But losing a healer isn't always an immediate loss, not the way it often is in WoW, because the tank can play very defensively and dramatically reduce the damage she takes, buying time for the DPS to finish the fight.