According to the new vid, you can see them as any class. But if an Operative ADS at it, it glows read and you can see it (maybe through walls). If an engineer couldn't see it he wouldn't be able to disarm it without an Op and that wouldn't be balanced.
This is why they don't detonate unless you step off it. Playing W:ET as an Engineer, it was quite common to crouch and walk through "mine hotspots" listening for clicks of mines you stepped on, and disarming them as you moved along - And Engineers or any other other class, couldn't see them unless they were spotted first.
However, they could be detonated by an explosion, whether they were spotted or not.
There were also the kamikaze Medics, who would run through while dropping medkits.
The mechanic they are using for mines really isn't a big problem - it worked it previous games just fine. I just think it is a new concept to a lot of you, and so you are all assuming the worst and "Doom Saying" before even trying it out.