I’ve updated my Mac keyboard shortcuts page for El Capitan. Not much changed really. While I was updating things though, I added a new keystroke. It’s not new in El Capitan, but was surprised I didn’t already have it listed.
To turn display mirroring on or off (when connected to an additional display) you can press Command–Brightness Down (F1).
Safari 9 changed the behavior of Cmd-1 to 9. By default it now switches to the first tab, second tab, etc. Previously it chose the one of the first 9 bookmarks in the Bookmarks Bar. If you prefer the previous behavior, go into Safari > Preferences > Tabs and uncheck Use ⌘-1 through ⌘-9 to switch tabs.