Highlights include Sliding Sync (instant login/launch/sync), Native OIDC (industry-standard authentication), Native Group VoIP (end-to-end encrypted large-scale voice & video conferencing) and Faster Joins (lazy-loading room state when your server joins a room).
For now, you need to run a sliding-sync proxy besides synapse. They’ll integrate it properly, but for now the focus was on getting it to work.