Mine is that some people are sensitive to carbs/sugars... and if you're cursed in that way like I am, then totally eliminating bread and sugar can make a huge difference.

I also recently discovered that eliminating wheat can make a huge difference. My brother did exactly that, and the weight has been melting off without any other diet changes!
http://www.wheatbellyblog.com/

What's your #1 nutrition advice for others?