Recently I started help Dave playtest a game he's designing and I received a submission from another designer who would like me to publish their game. It's been an interesting experience, and until now I've only playtested my own games, and been on the receiving end of feedback rather than had to give it myself. It made me think of what I wanted from playtesting feedback, so here's my thoughts on reviewing a product in development. Feel free to disagree (or back me up) in the comments.
The designer will have invested heavily in their game, they will have given up lots of their time and effort to get it to the stage it has finally reached. They now think it's good enough to play with others, and as you would expect they would like to hear that you think it's good too. However, they need to hear its weaknesses. You can get blinded to the bad points of something you're heavily invested in - it takes someone from outside to see the wood from the trees.
So slag it off? NO! The key is constructive criticism. Look for both good and bad points and relate them both. Remember your opinion on the game is just that - a personal opinion. Try to separate the game from the designer and avoid personal attacks - these won't lead to your feedback being considered.
Having said all that, be honest. If you don't like something say so - and provide reasons why. Suggest alternatives - they may well not be used but they are more constructive than just slating it.
I think it's also vital to have a pen and paper handy when you're playtesting a game too. That way to can make a note of things while playing the game without breaking the flow of the game too much. You can discuss the pros and cons afterwards without have to keep them all in memory while you're playing the game.
In other news I finally got some gluing done last night - enough to finish off that batch of four games. Once they are finished I should have some stock again.