People need to relax until more details are available. By my best guess, "main tank" simply means most likely two things

1) Having slightly better self-protection and
2) Having more "counter attacks" than off-tanks (use a "defend" skill like Shelltron, get hit, able to do a counter skill)

If that's the case that does not mean you won't be able to tank as an "off-tank", you just won't have the "counter attack" play style. And with off-tanks having less self-protection, yet still able to survive tank busters, it could mean healers would need to spend 1 or 2 more GCDs to heal after a tank buster, reducing the advantage of taking two off-tanks for higher damage.

With this, I don't mind this change, quite the contrary. We'll just have to wait and see until more details are available.