I think a really big one is the “dislike” factor. Clinton was really disliked by a lot of people, and importantly, by a far larger number of “independents” than Biden is. I am also assuming that the Greens will have far fewer votes this year as people rally to Biden to get Trump out.
[Anecdote warning]I do know a few American far lefties in Japan and they were staunchly anti-Clinton while complaining that too many people were supporting Trump (“hello! He can’t win if you all vote Clinton!”). This year they seem much more to be selectively praising bits they like that Biden has said, and saying that once the election has happened, then they can start holding Biden to account, but that in the meantime, they have to get the racist, authoritarian clown out of the White House.
So there *do* seem to be some tangible differences this year.