I loved that one huge mistake he made, because it's playing off the huge mistake he made last month at WrestleMania. They're starting to make the cracks in Cena's armor show, and if it wasn't for a mistake of Brock's own, Cena would have another self-inflicted loss on his record. It's a nice little piece of storytelling that makes the outcome of the match, and Cena's "super" comeback, seem OK. To me, anyway.
Having Cena going over wouldn't be such a big deal if it weren't Brock's first match back, and if they hadn't set up a completely different story. I'm very big on following through on the booking you set up, or if there is a change, it had better be for a reason.
They set up a number of great possibilities, and now that's all they can be: possibilities.
This should have started a "dark period" for WWE, similar to the McMahon-Helmsley Era where the bad guys were in charge, and there was noone to stand up to them. Brock and Johnny could have taken out anyone, HHH included, and basically turned the place inside out. You could have also had Brock take the title from Punk, and brought some prestige back to the belt when someone finally takes it from Brock.
As it stands, the beatdown was just a hiccup, a bump in the road.