Donald, what does a typical playtesting session run by you look like?
(Do you explain to playtesters what you are looking for before playing or just let them play? Do you have playtesters fill out a detailed "Donald X. Exit Survey" after, or have them verbally give you their playing impressions?)
IRL I would deal out 5 cards from the set being tested, and 5 from some other set (sometimes a different ratio for small sets). Sometimes instead it would be all 10 from the set being tested. After each game I would replace just 4 cards, rotating which slots changed. Sometimes a particular card would need testing and I'd just make sure that was out. Sometimes a particular card would need changing and I'd say "this game this costs $6" (usually marked with dice) or sometimes, with experienced players only, "this game let's try some new thing I haven't printed out yet, it's a treasure costing $5 so we'll use Venture to represent it."
Online we would typically play, 5 cards from the new set, 5 random cards from all sets. Sometimes a card would need focused testing and it might be, this card plus 9 random cards. There was even more focused testing; Crossroads, Margrave, and 8 cards from Hinterlands. We tested recommended sets.
Sometimes you would be limited as to what you bought. This game, these two players can't buy this card, these two players have to buy it (some players are awful at having to buy something so I leaned towards that slot). Maybe we decide after the opening split in case the card costs $4 and someone has $5/$2. I've specifically tested openings; okay you get $5/$2 and open Soothsayer.
It's possible we sit there chatting about the game afterwards, or just move on to the next one. I ask people what they think. I say what I think. For online players, there was a forum, and we would talk about whatever there was to talk about, and sometimes I would say, could everyone say a few words about every card. We would post game results and say a few words; lots of games happened that I wasn't in.
With new players I am often just looking at, what did they ask questions about; what has to go in the FAQ, what should be reworded. What did they enjoy.