In my view, the biggest difference between Facebook's Libra and the Bitcoin is that while Libra is a Stablecoin, Bitcoin is not!
lol, that is not even important

the biggest difference is its centralization which means you have to sign up with Facebook in order to be able to use this coin and also since they are controlling the network (mining and nodes and supply and....) they control your money like a bank does. so they can decide to "close your account" if they see fit, like for example if they suspect your activities.