I have that same reservation even after seeing him in action. We have to win this election, and he will have trouble getting the votes of Blacks and Latinos if he made it to the general. Those are two communities that are behind the curve on LGBTQ acceptance, and we need their votes to win. I wish it were otherwise. Of course that probably means he can't win the primary too, so maybe it's a moot point.
He'd be a good VP behind Harris though. Would love to see him go up against Pence in the debates.
I think it's more than the usual "likeability" aspect of charisma. He has a real talent for explaining complex topics in easily digested, everyday language.
Sometimes a highly intelligent person will come across in a way that puts people off, and he doesn't do that. He's mastered the art of sounding like the proverbial guy you'd like to have a beer with, and not a Brainiac (which he obviously is).