[Lycrist]

I'd like to think that the tank would be a pivotal part of the group, just as a healer is, and losing him/her, while not a definite loss would really make the other team members have to pull some strategic gameplay to survive.

The tanks survivability, hopefully, would be as necessary as a healers.

That said, I really hope they make this true for the spellcasters and offensive fighters.

In almost every MMO I've played, the tank and healer classes are almost always a necessity, but you could generally go without a spellcaster or an offensive fighter.

I'm sure Sigil will have some creative ways of implementing each job type though.