physically? It can be anything- dress sense, appearance, whatever.
I get the eyes thing a lot (well, at least compared to everything else). I was walking through the Arndale Centre and a guy selling nail products walked over, stopped dead when I looked up and acted all shocked: "I'm sorry...it's just, you have such amazing eyes!" Sorry pal, that's not going to make me spend £30 on cuticle cream.
Also when I was in Canada, I got compliments on my clothing choices, but not here- overall people dress nicer here, and I look average rather than standing out.