I voted this morning, using the no-questions-asked absentee ballot. Virginia starts counting them Sept. 28, and I mailed it in the condo building. You had to put the ballot inside a second envelope and fill it out, every field. But media has told us that you do not need to have a witness. (The envelope tells you if you need an extra id). Patricia Sullivan explains in the Washington Post. Zachary Wolff writes further on CNN.
Now Trump is already starting to jump on little problems with mail in, as with an incident in Luzerne County, Pam as explained by Marshall Cohen et al on CNN. Stephen Loaiconi from Sinclair has a somewhat more disturbing interpretation on WJLA.
The Daily Caller reports a disturbing incident in Texas.
In sum, because mail-in may happen in unprecedented numbers due to the pandemic and due to the complexity in filling it out for some less literate people, there could be more problems.
Fareed Zakaria has an article explaining how Trump could stay in power without winning the election, by creating so much legal confusion (with the new SCOTUS appointment) that state legislature get to choose the electors. They happen to be Republicans in swing states.