The most notable changes:

• bitcoind used to listen on 127.0.0.1:8334 by default. If you use Tor for incoming connections, you have to manually specify bind=127.0.0.1:8334=onion in config
• unix sockets can now be used to communicate with Tor or other proxy, and MQ traffic.
• New mempool policies has been implemented to patch some attack vectors for chains of unconfirmed transactions, especially in relation to lightning network channels and similar contracts.
• TRUC (Topologically Restricted Until Confirmation, BIP 431) can now be used with transaction version 3 (now considered standard) instead of RBF.
• Full RBF (Replace By Fee) is now enabled by default
• RHEL 8 and Ubuntu 18.04 are now unsupported due to minimum required glibc version bump.