16. "Options" menu!
That's the thing most devs worry least about but should work on pretty hard.
Options menu must be accessible at any point because player doesn't know if current options are ok yet.
a) Customizable controls - I did this in my game, working on a harder part right now - configurable gamepad!
b) If game is cpu/gpu intensive but you love the look of it, try to make some options like effects switching, low and high resolution modes, particle switches, toggling physics if physics don't play a huge role in the game, etc.
c) Options must be easy to understand, there are three things to think about:
1) Amount of text on screen(I have problems with that sometimes)
2) Font, size, position components, overal design
3) Tree menu - how you sort options by categories
d) Keyboard shortcuts may come in handy sometimes, like volume adjusting or item equipping or something else.