I have brewed an oatmeal stout that had an acidic/salty taste to it. I am pretty sure there was scorching going on. I talked to some professional brewers who said the chocolately flavor in stouts, when overheated to an extreme point, can cause that chocolate to turn bitter.
A gentle boil should look like a bubbling brook. You should see movement on the surface of the liquid, but not so vigorous that you may risk getting splashed with some stray liquid. A violent boil has bubbles that are so extreme that I have been burned on my arms just by standing too close. I brew indoors, and some violent boils resulted in my basement being turned into a sauna. Smelled awesome and my friends loved it, but the resulting beer was baaaaad.