It seems that Windows 10 for phones might not feature a lot of UI changes but mainly UX (user experience). Leaked screenshots of a Chinese version of the OS shows basically the same appearance with minor improvements.
One new functionality, though is Microsoft making provisions for easier typing on larger screen phones with the introduction of one-handed typing. The keyboard can be moved to the left or right edge of the screen to make it easy to type with one hand. Also, voice input directly accessible on the keyboard.
Another new feature revealed in the leak is a separate recording app, which enables users to simply record voice input and save the end results for sharing via email and OneDrive. Unfortunately, it does not record phone calls just yet.
Another last improvement is the UI redesign of the alarm app to like the type found in Windows 8 and Windows 10 for PCs.
I am pretty sure that as the release for the Windows 10 approaches, we will get to learn more about other new features.