Could you go into more detail? I'm going to ask a friend of mine with a PhD in rhetoric what she thought as well.
Putting on my Aristotelian hat here. A very broad Aristotelian take.
Looking over the Twitter chatter, this is one of the all-timers. Not convinced of that. It was a fine speech. He only needed to do fine. If I showed this in my public speaking class, I would expect my students to have ample amounts of praise and criticism for this. That's Twitter for you.
I was struck by how short it was - likely a product of the production style (no audience) - which will compress delivery time but also change the message since you don't have an audience to seek a response/respond to. That lack of audience to play off of, and its effect on the speech I thought resulted in the attempt to compress a few, key lines of thought (civil rights, pandemic, climate change) by centering it around a single appeal to ethos (that his personal character is superior to the opponent, or rather, that the other's ethos is non-existent).
The lead up to the speech balanced emotional, character, and logical appeals a little better - the stuttering kid, the grandchildren speaking highly of him, some args from Buttigieg and Bloomberg. Makes sense, they're recorded. But, the produced set pieces seemed to pull on my heartstrings and make better arguments than Biden did, imo. I'm not saying his speech completely missed this - just that it can be difficult to confidently work in strong emotional appeals and reasoning when the crux of your speech is an (even if it's warranted!) attack on another's ethos. Put another way, a slightly more talented speaker/speechwriter team (always co-constructed) can make you dislike the opponent without even mentioning the opponent. That didn't happen here.
edit: repeated myself