Can I make iOS 8 look like iOS 6? This has to be my #1 wish for iOS, bar none. I swear the look of the GUI in iOS 6 fits my brain far better than that of 7/8. It's not just a matter of cringing every time I look at it; there is no contest in terms of my productivity being much higher on iOS 6.

Is there any way (via Cydia etc) to make all apps (or at least some of them) use the old GUI style?

I know there are lots of apps in the app store that simply haven't been updated and so use the old style so it seems that a small amount of runtime injection could achieve this.
Try this: http://cydia.saurik.com/package/org.thebigboss.ios6completetheme/

I'm not yet jailbroken but plan on doing it soon so I can't garuntee this works