The most important quality for a WWOOFer to have is to be able to fit into the very different lifestyles that the hosts lead. You are expected to be interested in what they do and help. They would like some help with the chores (such as cooking and washing up) and apprecfiate it if you did not create more work for your host. Hosts are all very hardworking people mainly living on a tight budget and you should accept this fact and respect it.
WWOOFing is NOT a cheap holiday and way to travel. It is a voluntary organisation set up to teach people about the organic movement and give them hands on experience and to help out organic producers as organic production is very labour intensive. If all you are looking for is free accomodation on your holidays WWOOF IS NOT FOR YOU.
Always respect the dates you arrange with your host, if you have to change them for any reason you must give your host ample warning so that they can make other arrangements. Please don't just not turn up, this can cause major problems for your host (who has probably turned down other people as they were expecting you to come). Please be responsible and thoughtful and take your responsibilities seriously. If your stay is not working out for your host he can always ask you to leave, although with good communication between host and WWOOFer it does not usually need to resort to this.