It's hard to find an example of any convention where the acceptance speech didn't get good reviews (especially the Dems given the media bias). Al Gore, John Kerry and HRC all got good reviews. These speeches are written well in advance by talented speech writers. They are designed to be light on detail and loaded with feel good fluff and red meat for the base. The candidate has all week to practice delivery, timing, body language, etc. with professional speech coaches/consultants. The entire things is totally scripted by an army of consultants. It's almost like a Hollywood production (insert favorite movie president speech).
Biden's speech was good in the following ways:
1) he could actually read the thing and didn't have any huge blunders (a very low bar. A blooper in a canned speech off a teleprompter to an empty room would be bad even for Joe).
2) He made a lot of warm fuzzy comments (like "I will be everyone's president"), if you total ignore the way Joe and his party have operated for the last 40 plus years.
3) He attacked the thing that most potentially persuadable voters don't like about Trump, which is his personality and was semi successful at coming off as more statesmen like. (This again ignores his past history completely--but if you were just watching the speech in a vacuum, he presented well [enough].)
But, he did very little to nothing to create enthusiasm. It's like the Dole convention in 1996. He was a veteran senator. He had served his country well. He had a nice family, etc. etc. Even he gave a good convention speech. And he got a post convention bump (double digits) that Biden thus far has not seen. But, at the end of the day people were excited about Clinton. Dole generated little enthusiasm and was not going to unseat a charismatic incumbent with solid base support.
Biden's "homerun" was literally that he didn't forget where he was or who he was.
The character attacks won't really change anyone's mind. HRC attacked Trump in 2016, the media has been doing it 1,000 times a day every day of his presidency. I don't think the Covid stuff will go far either. People are getting fed up with lockdowns. Europe is facing huge spikes recently (let's see how they can blame Trump for that), and the blue states botched it worse than anyone. Biden had no specifics other than an unenforceable national mask mandate. Voters will realize the intellectual dishonesty of criticizing Trump for not locking everyone up like South Korea or New Zealand, while also blaming him for the jobless numbers. Notably, the economic toll in places like the UK and Germany is far worse, and they are still spiking. The economy was humming along before Covid and all demographics were doing better than ever economically.
It seems like Trump has an easy case to make about who is better to restore the economy. This isn't a Bush 41 problem ("It's the economy stupid") where he can be blamed for a global economic crisis caused by a pandemic. If Covid were a US only phenomenon then maybe voters would hold Trump accountable. But, I just don't see that happening. Biden ignored the looting and lawlessness that turns off most Americans (see first time gun purchases and multiple threads on the Outdoors board about not being able to buy ammo). The door is wide open for Trump next week and throughout the rest of the campaign cycle.
If Biden's speech was a homerun, Trump can carry on the themes of his Mt. Rushmore speech and make his a Grand Slam.