...disclosure agreement? I know this has to be done if they are using a realtor, but if you buy a house directly from the sellers, are they required by law to tell you if anything is wrong with the house (bad plumbing, standing rainwater in the backyard, etc)?

By law they have to disclose EVERYTHING!!! If they have a problem with filling out the form, that should be a BIG red flag. Consult a Realtor to represent you. She/He can take care of all that for you and negotiate the best price on a home as well as provide information and guide you while finding the right property. You should never purchase a home without a RE professional. Their services cost the buyer nothing and you have someone looking out for your best interests.

Yes, they are required to disclose. Even "as is" sales require disclosures if the owner is aware of problems. Keep in mind that what you might consider a problem might not be something they consider a problem (like noisy neighbors who ruin your quality of life...you'd want to know that, but you'll have a hard time finding someone to disclose that). Most disclosures are related to structual defects, problems such as termites or water damage, dry rot, roof leaks, bad plumbing, not up to code electrical, etc. Make sure you have the house thorougly inspected, make sure there was not been any unpermitted repairs/modifications done to the home.