I noticed this topic brought up - and I figured I would give my take on this issue, but I would do it in a new message so that it would be more organized.
Ancestry.com, FamilySearch.org, and Genealogy.com does have some acceptable sources. However, they also have sources that are far from accurate or acceptable.The trick is to know the difference.
When sourcing information in your tree - you want to make sure that every source is acceptable and has an ounce of proof to it.So how can you tell?
Sources that are actual documents, such as census records, pension records, military draft records, ship manifests, etc, are good. These are the types of records that you want to strive for.
You don't want to use sources that are compiled from gedcoms or family trees that people submit to a website.Not every family tree has sources, and not every family tree has accurate information which is why it is important to only use these as a GUIDE and nothing further.Databases to avoid include Ancestry.com's OneWorldTree and FamilySearch's Individual Record because these are all databases compiled from submitted family trees.
When a database lets you look at the image that correspondes with the index, then it gives you a chance to interpret for yourself what exactly is in that record.