Having run playtests I can tell you that I like it when I get to hear from my playtests what games they like to play, how long they have been a gamer and in general, talk about things other than the game we are testing.
This is not always possible because playtest events can get very busy and the designer may not have time. But if you find yourself in a situation (maybe even during the playtest) where you can chat with the designer about your gaming background or your likes and dislikes; this will be valuable information to the designer. It will help the designer give weight to your feedback. Even if the games you are testing is not the kind of game you like, maybe there was an element you really liked and the designer can look at that and think about how to give their game a wider appeal.
If you give the designer an idea of what games you like to play, maybe they have a game they are working on they think you would like and they can give you an advanced copy of the rules for you to look at.
Keep in mind, not all conversations with the designer need be in person. Lots of designers are on twitter and probably on Board Game Geek. They should have contact information at the event or you can ask them for it and you cans trick up a conversation online. There are benefits to both you and the designer is you keep in touch with that and designers love to talk about their games all the time.
If you have any comments or questions, leave a comment here or email Chris at firstname.lastname@example.org