I think iOS's top needs, with regard to UI, are, yeah, a revamp of the lock screen, a simplification of the notification center, and easier access to commonly used settings. More use could be made out of the app switcher, too, and the Spotlight screen.
And yeah, a UI chrome refresh with a flatter design would be nice for sure. Skeuomorphism in one sense of the term is fine -- drop shadows, for example, are a useful indicator of which elements are "in front" of each other, as are the concept of buttons, but that's very different from the amazingly gross leather/torn paper stuff, which needs to go.
I think it's much more important, though, to get apps to be much better at talking to each other and to fix iCloud data syncing. I think revamping the way apps handle documents would be a good thing as well so they could be more easily opened and edited by multiple apps instead of having to make new copies of themselves each time that occurs.
Some nice tweaks would be simple stuff like folding the Voice Memos and Notes app into each other, or getting rid of the Compass app entirely (since it was really just there as a tech demo for the compass inside the iPhone 3GS and its functionality is fully contained within the Maps app).