I really haven't come across too many ethical issues in my genealogical research, but there two that I believe are important. I won't talk about copyright laws as Steve Danko and Footnote Maven have already posted on this issue. Personally, when it comes to copyright issues, I think it is just better not to post what ever it is on the internet. But I digress; there are additional issues that bother me.
Online family trees, like Ancestry's World Connect, without sources bother me. I personally don't believe that citing a gedcom is sufficient enough as a source of where one personally obtained the information on their tree. I realize that I could always e-mail the tree's owner; however, I have run into situations where a person will have listed one of my ancestor's on his or her tree, and upon e-mailing that person, I learn that the tree's owner does not have any further information or where he or she has obtained that information. Or, I am unable to contact a person because there isn't an e-mail address listed. I'm sorry, but if someone is going to spend the time and effort to post his or her family tree on the web, he or she might as well spend the time to add citations on where he or she has found the information. The time spent on researching one's genealogy is fairly equivalent to doing research for a history paper or a history book. I realize that not everyone is a history major, but in every field that requires research all of the final papers have to have citations and information on where one has obtained his or her information. Any information that is not part of the public domain has to have a citation. It has to be verifiable. The citing format that I use is the Chicago Manual. I use this format for my citations in my history papers and since I'm familiar with this format, it is more convenient to use.
Another issue, with is really a sub-category of the above, is the plagiarism of other people's research. Yes, it is plagiarism if you copy someone else's research and then post on your tree as your research without acknowledging that other person's information. I realize that you can't take information on the web as gospel truth. My problem is that some people have copied other people's work, and having stated that the information they have posted is not their own. Why? It is downright dishonest. I realize that the best way to prevent people from stealing one's research is to not post it on the internet at all. I don't have my family tree online, and if someone has given me information, I cite who's work it is.
Another reason why I do not have my family tree online is that I want to minimize the risk of identity theft. I do not want to make it easy for anyone to steal my or a relative's identity, for that matter. And as I stated briefly before, it is better to not post something to the web if there is a risk that a person could end up in legal trouble or end up with a family feud. Or have one's identity stolen. Sometimes it is better to be safe than sorry.