In a recent episode of Game Design Roundtable, Dirk Knemeyer, David Heron, and Rob Daviau discuss the nature of all sorts of feedback from playtesters all the way to post-release reviews. The bits specifically relating to playtest feedback were most interesting to me and I thought I'd list them out here for future reference.
Matt Leacock asks testers to just record sessions on video, no written reports. He'll just watch the video at 1.5x normal speed and look for moments when players check the rulebook, check their phone, or just check out completely.
If you can't record video, be ready to listen.
Rob Daviau lets playtesters talk it out, even if conversation drifts to solutions that he has no intention of implementing. He just lets playtesters keep talking because they'll inevitably talk around the real problem.
Communicate your design goals.
Watch out for playtesters who evaluate a game against their own preferences rather than your design goals. "This soup is an awful cake," is not the most useful feedback.
Hit the road.
Finding local playtesters is difficult, so you have to really stretch what you think of as "local." Be prepared to log some miles to find willing playtesters at game stores in neighboring towns throughout the week or month. (I log my own miles for tax purposes, and I can tell you they really add up.)
Be physically prepared to take feedback.
Make sure you're in the right mental state to hear playtest feedback, otherwise it might blow up. I can tell you from personal experience that by the end of a long playtest day without much of a break, I can get very catty. Rest, hydrate, eat, pace yourself.
You can listen to the rest of that episode here. It's really good!