MacOS – Restore option-left and option-right in MacOS terminal – What is the action sequence

The same thing happens on my 10.9 and 10.8 VMs. Control-arrows and shift-arrows don't do anything by default in bash. Where were you even using shift-arrows to select text? emacs or vim?

If you want to use control-left and control-right to move between words in bash, add these lines to ~/.inputrc:

"\e[1;5C": forward-word
"\e[1;5D": backward-word

To use shift-arrows in Emacs, map shift-up to \e[1;2A, shift-down to \e[1;2B, shift-right to \e[1;2C, and shift-left to \e[1;2D. If pressing shift-up results in a message like <select> is undefined, try setting TERM to xterm-vt220.

I think the OS X Human Interface Guidelines are the place you're looking for. They state that there are Apple-reserved shortcuts (e.g. cmd ⌘+space) and also “expected behaviours” (e.g. cmd ⌘+c) one should respect.

While they don't explicitly state the priority, it suggests itself that the System always should have priority before the frontmost (active) and then background applications.

From my experience, you’re on the safe side with multiple modifier keys (i.e. cmd ⌘+ctrl+shift+…).

As an aside: regarding your specific example (assigning a shortcut to open a new terminal) there's also excellent apps for that, TotalTerminal (Terminal.app extension) and iTerm 2 (Terminal.app replacement) come to mind