Immutability - The ledger can be permanent, transparent, and unalterable history of all transactions made.
In smart contracts, code is encrypted, enabling autonomous and trustless transactions can be made, and parameters can be respected (i.e capped supply in BTC).

Decentralization - Rather than putting our trust in a single company which will control our data, we should trust the systems, networks, and algorithmic rules that are distributed, transparent, and open-source.
This enables censorship-resistance, on a worldwide trustless ledger.