Just another modern city.
I do not have much impression on Frankfurt, as it's just too developed as a modern city. There are many high building and skyscrapers. For me I didn't find much culture value over there. But one good point is that we came during Christmas season, so the Christmas markets there provided delicious food!
Insider Tip:Frankfurt is not worth a long stay though! Wandering around the city if you are transiting there for a few hours sounds nicer than a long stay.