Start with the fact that he is too damn old (so are Trump and Bernie). Biden's ship took on some water after Harris hulled him in the first debate. It was a prepared but rather unfair attack, since Harris has subsequently waffled on whether she supports the kind of involuntary busing Biden opposed.
That attack did not bother me much, but Biden's muddled and inarticulate response did. Trump is a master of the attack from left field, the over the top smear, and if Biden is this slow on his feet on a topic he should have expected, I can imagine Trump running all over him.
I also find Biden's threats to punch Trump childish, but it could be the biggest pay-per-view ever.