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).
One of the items at the top of my lists, and I suspect at the top of the list of many reporters, is the built-in ability to do screenshots. This has now been confirmed as a feature for Windows Phone 8, and it would be great for us to have this back-ported. But what else would you like to see?
You can pitch in with your ideas over on the Uservoice website.