Bitcoin is simply known as electronic currency which has no physical form where as fiat currency is known as physical currency. Bitcoin is a decentralised nature currency which does not under control of any kind of authority or government where fiat currency are under control of many government. Fiat currency are more stable then that of bitcoin and it is not complex as that of bitcoin. Fiat currency can be duplicate because of their physical form but bitcoin cannot be duplicate because of its digital in nature.
Yes, there are a lot of differences between them and that's why replacing fiat with Bitcoin sounds perspective. The only critical problem most of you admitted is lack of physical form of Bitcoin, but I believe that we can find a solution for this issue.