For those who are developing games and have a playable demo I highly suggest watching this video. It goes into detail about how to properly playtest and gives some great tips along the way. This is an absolute must see for developers who haven't play tested before. Even if your game is super simple you'll be surprised at how much you can learn by letting someone else play your game.
Find a family member, friend, or grab a stranger of the street and sit them down in front of your game. Make sure you listen more than talk and only speak up when they start to get frustrated and can't figure something out. Most of all have a notepad handy so that you can jot down information as they are playing. Thanks to The Game Prodigy for this excellent video.