As you know, Windows Phone 8 is a generational shift in technology, which means that it will not run on existing hardware.
BUT we care deeply about our existing customers and would like to know SPECIFICALLY what features you would like to see on 7.8 and vote INDIVIDUALLY for the one that you want most (vs. one large grab bag of features).
So what features can you vote for? Some of the most popular so far include custom alert sounds, screenshot support, the ability to 'close' an app from the multitasking screen, controlling the ringer and media volume separately, and plenty more. Go ahead and make you voice heard by voting for features here.
Source: Windows Phone Uservoice