There are some mild spoilers forthcoming.
Tarring and feathering is a much more gruesome act of violence than I'd ever considered. Not the feathering, so much; it's dumping boiling tar on naked human flesh that upsets me (if you want to read something harrowing check out the Wikipedia entry for tarring and feathering, and if you think you have the stomach you might also read about pitchcapping). The tarring and feathering is the most graphic example (though not by much) of John Adams' emphasis on the corporeal brutality of life around the time of the American revolution.
For a lavishly mounted period drama the series (for the most part) doesn't skimp on these details. Medicine often seems as terrifying as sickness. Smallpox inoculations, blood lettings, amputations and a mastectomy are all depicted with horrific frankness. The show also notes the constant discomfort George Washington felt as a result of tooth problems. He was apparently in awful pain when he sat for the portrait reproduced on the one-dollar bill.
In an early scene John Adams teaches young John Quincy Adams about manure, about how it should feel and how it should smell. When the son declares his desire to be a farmer, his father tells him it shall be law school for him first. The juxtaposition in this scene, between harsh physicality and enlightenment, is a theme not just of the show but of the country's long narrative, as well.
A few details are spared. Though the show periodically makes slavery an issue it elides the associated violence or degradation. This isn't so much a problem, to me. Surely we have something close-enough to national consensus on slavery nowadays that we don't really need to exploit that white-on-black violence by graphically staging it, right? (Which is not to say the lives of Frederick Douglass, Harriet Tubman or Nat Turner couldn't also be mined for a spectacular miniseries.)
There's also not much sex in the show, but perhaps Abigail and John weren't so ahead of their times as to leave much of a record of that aspect of their relationship.