I agree with you that generally people seem better looking the more I like them. I've had the experience several times where I thought someone was ugly initially and then when I got to know them I changed my mind -- and vice versa.
I have been told that I am good looking, and I will admit it is something I like to hear.
Also, I think a large part of being good looking is being healthy and fit, and yes, this also includes being thin. I'm not sure there are any "innate" good looks that could make a person look attractive at 300 pounds.
And I think teeth also matter, crooked or rotten teeth are a sign of poor health and/or low social status (mine are a bit crooked).