I think the best experience I've had was on the Southbound Adirondack (like the Carolinian, the Adirondack is formed of Amfleets), which doesn't have business class, but on the day I traveled on it, happened to have a business class carriage stuck at the front of the formation; one of the two carriages that they like to put all the passengers bound for Penn station into. I queried this with the conductor standing by the train as we were boarded, who told me yes, the markings made no difference, every carriage was coach.
So in I stepped, and having been anticipating 11 hours on the same accommodation as the Downeaster, I was delighted to find more comfortably padded, reclining seats, with far more legroom, and proper footrests. This makes a big difference when you're traveling in coach for that long.
It was, in the same way, 2+2 seating, and, having started off with a quite dull business type sitting next to me, his girlfriend asked if I'd be willing to swap with her so they could sit together - which, of course, I gladly did, rather than see them separated for the length of the journey - I ended up next to a friendly student of about my age, who was at the same year of university as me, and who knew of, and liked the same semi-obscure indie bands as me, proving the principle that Downeaster conductor Jim told me:
"You meet the most interesting people in coach".
I still boast about this journey to the friend I was heading to in New York at the time, who pays hideously overpriced fares to travel business class on the NEC's amfleets.