The more I think about it the more a larger screen size appeals. Most of my usage is web browsing, e-mail, etc. while out and about/at work, so a larger screen would help with that. A wider form factor would also make the phone more comfortable to use in general as I've always found iPhones a bit small for my bear hands.
I don't want something too big of course, but something similar to the concepts that have been posted online would be perfect. I can't really think of much else that would have me really wanting to upgrade from the 5 at the moment.