From what we've seen in videos, looks like it's a short roll. Probably the general length of a human body laying down. The Rogue had one where they leap forward into a roll which seemed to move them much, much further. And we've heard (or seen, I forget) of mages getting a (fake)Teleport that lets them invisibly/hasted move from one point to another to avoid danger. I would imagine your immune during the bulk of the animation, at least at the first part to be able to avoid damage.
As for Block and Slash no, if you read it, it consumes stamina per second. I doubt you can do 'anything' while its active. You take a defensive stance and don't do anything till you stop using or till you block. Basically its a full defensive mode, meaning you can't do square while your using it. Abusing it, would ultimately destroy your Warriors DPS while constantly draining your stamina. That's.. that's not really what I'd consider OP. Then again I also kinda feel it just drains stamina till you block a single attack then deactivates. Again, your doing nothing till you block said attack so DPS loss for the sake of blocking a single attack. I'd also imagine some larger attacks (such as Ogre 2-arm slam or bullrush, or boulder throwing) can't be block/countered like that.
Now if it was a sustained that allowed you to auto-attack and make attacks while consuming small bits of stamina, then pairing it with the group combat stamina gain from hits (10% per additional target past 1st you hit, so 2nd, 3rd so on) I could see that being EXTREMELY overpowered. But doesn't seem to be how it works from what I could see.