I honestly don't think it's going to be as hard of a mt/ot split as many people are assuming.
From what has been shown so far, my guess is that it will likely be that the "mt" tanks will just have a few extra things around managing direct single-target damage, while the "ot" tanks will have a few extra things for managing AoE/multi-target damage.
For example, as shown at the panel and already pointed out in the thread, PLD can redirect the damage from a tankbuster hitting the other tank to themselves with Cover and still trigger their retaliation ability since they used Holy Sheltron on themselves. So, the PLD can still use their "MT" aspect of managing direct damage in the "OT" position.
Where it may be "less efficient" is simply that if the PLD had been in the "MT" position, they wouldn't have had to target the other tank and use Cover.
Since an example of one of the "OT" jobs hasn't been shown yet, it's hard to determine how exactly those will play in terms of how they may be helping manage AoE damage for the party. I really hope it's not just giving them an extra ability or button to press that just does it, that would be super boring.
That actually brings me to my potential biggest concern, which is disparity of fun and engagement between the MT/OT split.
They highlighted that the MT tanks will have this new retaliation mechanic based around well timed defensive abilities. That's super cool and will add extra engagement and skill to playing the MT tanks. As we haven't seen or been told much about the OT tanks, we don't know if there is any new game-play around their OT role similar to the retaliation mechanic of the MT role. It would really suck if there wasn't.