Based on my experience, I'll detail three negatives I have observed:
1. One of the virtually unavoidable negatives of homeschooling is the barrier of experience that exists between homeschoolers and 98% of society. Homeschoolers will never experience the traditional classroom environment and everything that goes with it. Obviously there are aspects of homeschoolers' reality that are not experienced by those who have attended "normal" school. However, there is not a sense of "missing out" or "being different" for non-homeschoolers, because the majority of the population attended public/private school.
2. A negative that can be more easily avoided than the first is the potential for close-mindedness. While this is a risk posed by any mode of education, certain aspects of the homeschooling experience are especially conducive to the development of a very narrow perspective on life. For example, in a "normal" school there are multiple teachers, so the viewpoints of one are easily challenged (in most cases). In contrast, homeschooling involves (usually) only the parents, allowing a limited exposure to to different perspectives. If parents deliberately expose their children to different viewpoints this potential negative is clearly not an issue.
3. Lack of social development is a potential concern that may be overemphasized relative to other concerns (especially in terms of frequency of occurence). For example, the "stereotypical homeschooler" is lacking in the area of social development. This probably is not true in most cases. However, there is a huge potential for parents (esp. those in fundamentalism of various forms) to utilize home education to isolate their kids and, thus, create various social issues.
In later posts, I will relate these negatives to my experience.