Graphical screen lock pattern instead of PIN code
I think WP7 contains a superior user interface to other mobiles OS's.
However the screen lock code seems out-of-date and sub-par to the rest of the interface. Android allows you to setup a graphical pattern, for example, while WP7 only allows you to enter a PIN code.
A graphical pattern is more practical and user friendly that a PIN code.
I don't think this is a difficult thing to implement and would improve the user experience.
Make it just like Windows 8!