1. QUICK COMMANDS
This is literally a version of Ctrl + F on steroids. There's a Chrome extension for this, but Vivaldi has the feature built right in. Hitting F2 will open Quick Commands, and you can do everything from searching all your tabs and notes to pulling up things like your browser history. You can even toggle the tab and address bar within the same field, and keying in specific actions like ‘New Tab' or ‘Clear Private Data' will also bring up the relevant keyboard shortcuts.
This is a veritable master control panel of sorts, and once you get used to it, it's hard to imagine a time when it didn't exist.