It's a lot of grey area and the laws are not fully defined yet which makes the environment even more hostile. Even if you misdiagnose a patient and you're taken to court and the court finds you were NOT negligent, you're probably going to be dropped anyway by your carrier and probably get reviewed by the bar. Laws also vary from state to state.
It's interesting to note that a doctor or lawyer can provide professional advice via a book or even phone, yet the internet has not been clearly defined. By now it should be obvious to everyone that technology moves far faster than the legislative process. Furthermore, there seems to be nothing to prevent a person from publishing a book on "Home Remedies" which could possibly prevent a person from getting the medication they need and result in serious consequences for the patient.
You have to look at it in a manner which is more akin to a parent/child relationship than an adult/adult relationship. The professional community assumes that non-professionals are mindless children that will do or take anything as advised without any thought or responsibility whatsoever and the professional can be held liable for any damage or injury that occurs to the non-professional (patient, client, etc.) as a result of that advice. Someone could have an painful hangnail and if the doctor said "We need to cut that finger off." and the person did it... Who's responsible? Definitely not the person with nine fingers. The only thing the guy with the missing finger is guilty of is blatant stupidity - and there's no law against stupidity yet.
The only thing you can do is not give any advice other than general advice and refer everyone to a professional in any specific circumstance. Having an MD on here would be fine if he wants to write articles and post general statements but it would be extremely wise for him to refer people to a local doctor before giving them medical advice or treatment and never NEVER charge for any service.