For one, it is common to stare at people who make look different to you. When you get on a bus or a train or some other form of public transportation and you're wearing something a little different from the norm, people will often look at you. It's not rude at all in Germany to stare and look if something is a little different than one is used to.
Secondly, less clothing is much more accepted here, even for women. In the US, many times if it is too hot or muggy outside, men will lose their shirts, however, in Germany, it is acceptable for women to do the same. In parks, at beaches, and even sometimes on the street people can be seen with much less clothing than would be legal in the United States.
A third thing would be that almost everyone in the city dresses fashonably. While I have been here, I have not seen a single person wearing leggings and a T-shirt. Many women wear dresses or skirts with heels even though they will most likely be walking more than a few kilometers in a day. Men are much more fashionable in Germany than they are in the US (no offense). Many men will wear slacks and button up shirts or accessorize in a way that many US men wouldn't.