All of the provided responses highlight important characteristics of Romantic concertos that differentiate them from earlier types. However, if you had to choose the most defining difference, it would likely be:
Romantic concertos placed more emphasis on virtuosity throughout the piece.
While it's true that Romantic concertos were typically longer and involved more intricate interactions between the soloist and orchestra, the heightened focus on the virtuosity of the solo instrument is a hallmark of the Romantic era. This emphasis manifested in demanding technical passages and expressive capabilities that showcased the soloist's skill and emotional depth.