Dave Land:

tab moves between buttons
space presses focused button (with blue outline)
return presses default button (with blue background)
⌘+first letter on button presses that button (works sometimes)
⌘+. cancels
esc cancels (works sometimes)

I’d forgotten ⌘+first, and I don’t think I’ve ever known about space and ⌘+.. Definitely going to be using this.