He looks like an absolute beast who could literally kill you if you ended up in a fight with him, so he definitely carries a lot of credibility with him as a character, athlete and performer in the ring. He was/is rather awesome in the squared circle and he's a freak of nature to be as athletic as he is for a guy his size (he once did a shooting star press off the top rope. He botched it, but still.) He isn't the best on the mic, but when he talks only sparingly and even then says very few words (his last two promos were the first time he's spoken aloud in a year, for example), then his words mean a lot more.
It's the fault of the booking as far as Heyman needing to bail Brock out of trouble in the ring. That's just WWE putting their 'superstars' over a guy whose biggest claim to fame was being UFC champion and being the biggest attraction in UFC history. Him coming right in and jobbing to Cena right off the bat was obviously their way of saying WWE>UFC. I didn't have a problem with him acting like he was on the verge of tapping to Punk's Anaconda Vice though, it helped build suspense and drama into the match. It also emphasized the strengths that a guy like Punk could use on the larger Brock, by showing that if he could just get him down into a submission hold and slap it on tight, he could maybe defeat him.