I agree with solarion regarding this, I think something being raw is just someone not being afraid to test out new ideas even if they don't look the best. Something that doesn't conform to any sort of standards, a levle where the creator had a vision and they did their absolute best to pull it off. Even if the execution isn't the best, there's a sense of passion that is needed for something like it. And while 'raw' is a term that's kinda grown out of its normal scope, it's usually referred to levels like this that are on the more intense side, usually being very abrasive with ideas that nobody would think of putting in their level, and yet the creator did it without a second thought.

Basically levels that are really cool and intense despite not being the best looking, and usually incorporating some sort of original idea or design choice(s) that the status quo of levels usually wouldn't come close to.