Quoted replies (i.e., quote part of a reply or create an arbitrary quote block)
Code snippets
Message forwarding
Polls
Animations in the UI
Detailed custom theming
Chat room theming
A content index (e.g., view only the files, links, videos, etc that were sent in this chat)
Group invite links to people you don't have in your contacts
Channels (i.e., micro-ish blogging)
A nice bot API
Subjective UI/UX changes to put things in more reasonable places (e.g, why can't I right click on a chat to pin it in the desktop client, why is the Electron menu bar shown by default)
And probably several other things I've forgotten because ... basically nobody I know is still using Signal.