Standby Screen, swipe UI and form factor (size and curved edges) are for me the standout features on the N9 which make it delightful, but haven't been replicated in combination on anything since.

X Compact doesn't have AMOLED so can't do Standby Screen

When I jumped off the Jolla train I went to a Z5 Compact (boxier than the XC) and it was great, but I missed the standby screen from N9, and obviously in Android there was no swipe multitasking from N9/Jolla.