I mean all wrestlers could beat me up so I'm not trying to say some aren't tough, but you look at Brock and you legit feel fear that he may just snap and murder you.

It's a combination of the things you mentioned: you know how tough he is in real life, he has an impressive resume, he does things in the ring very few people can do and he does it with ease and charisma.

He can carry the Big show easily over his head, and do a shooting star press (well he could at one point, botch at WM excluded). He delivers and takes very stiff shots (remember that fight with Cena?)

He is not your typical giant monster character who just looks big but doesn't do much, he really does do it all in terms of fighting. He is just exciting to see do his thing, part of it is all of the reasons we mentioned, and part of it is just an intangible thing you can't put your finger on for why some guys just have it.