I think the girl in the thread opener was definitely attractive. Maybe she's not at the ideal weight for her health, but she looks good. I think people look best when they seem to fit well on their frame and are proportioned symmetrically. I find very thin girls with huge boobs unattractive (though as I'm not into girls it doesn't much matter). I also find men with very bulked-up upper bodies and thin legs unattractive. Slim muscular people usually look good, but at a certain point the body-builder type tips over to point where I no longer find it sexy.
The BMI guidelines are good in general, but there are a lot of people in the "overweight" range who you'd never guess are overweight by looking at them, even in a swim suit. I'm right in the middle of the "normal" BMI zone, but have to work out a lot to stay there.
My newest story! An Island Competition entry: The Mermen of Jibei
Here is my back catalog: Charlotte's Pretty Hot Stories