Absolutely nothing.
There are more important things in life to worry about than what someone else next to me "has" or "doesn't have". Also, I don't believe in putting other people down or laughing at them behind their backs, even if it is "just to myself." I wouldn't want anyone doing that to me, so I try not to do it to others, especially complete strangers I know nothing about. The only emotion I 'might' feel is emphathy. I understand people like to have 'nice' things, and I understand not everyone can afford a designer handbag. What if that is all that stranger in front of me could afford? A $20 knock-off? Who am I to judge her? I don't know what her financial situation is.
In regards to the controversy about fakes, I agree that the mass population do not KNOW what they are supporting when they buy knock-offs. But maybe they do and just don't care. Who am I to judge those people? In my opnion that's no different than the controversy around diamonds, yet people still buy them, use them, and enjoy them.