Might as well start out with the largest website dealing with genealogy on the Internet. This is a fantastic website in many respects, but buyer beware of the shaking leaf. Not all that is seen is true on this website.
A large weakness with Ancestry is their inability to police and monitor their content. They do a fantastic job with adding new sources to their website on a weekly basis. Where they fall short is with the shaking leaf system. Be sure to check before downloading. I am sure they do offer some success with this process, but I have found more errors than good personally. You should make sure things are correct before adding it to your family tree.
The many family trees that have been added to the World Tree project is endless. Problems occur when they are totally undocumented. Here again you need to check everything. Trees with out sources abound on this website. Many errors occur. Example would be following a family tree back into the 1700's and then all of a sudden the tree goes wrong. A person born in Virginia who lived his whole life there does not have parents who lived their whole lives in Massachusetts. Mistakes like this are all over this website.
Primary sources on Ancestry are great. Those submitted by other genealogist when undocumented are dangerous. The myth you can trace yourself to King John in 2 hours is not reality. It still takes a lot of work to have a great documented genealogy.