For anyone with an iPhone 6 or later, the update's Apple Pay Cash feature opens up the ability to send cash to friends and family over iMessage. While I didn't see the feature immediately turned on after updating my own iPhone to iOS 11.2, when it does appear it should work similarly to Paypal's Venmo service.
Other improvements and fixes listed by Apple include:
Improves video camera stabilization
Adds support in Podcasts to automatically advance to the next episode from the same show
Adds support in HealthKit for downhill snow sports distance as a data type
Fixes an issue that could cause Mail to appear to be checking for new messages even when a download is complete
Fixes an issue that could cause cleared Mail notifications from Exchange accounts to reappear
Improves stability in Calendar
Resolves an issue where Settings could open to a blank screen
Fixes an issue that could prevent swiping to Today View or Camera from the Lock Screen
Addresses an issue that could prevent Music controls from displaying on the Lock Screen
Fixes an issue that could cause app icons to be arranged incorrectly on the Home Screen
Addresses an issue that could prevent users from deleting recent photos when iCloud storage is exceeded
Addresses an issue where Find My iPhone sometimes wouldn't display a map
Fixes an issue in Messages where the keyboard could overlap the most recent message
Fixes an issue in Calculator where typing numbers rapidly could lead to incorrect results
Addressed an issue where the keyboard could respond slowly
Adds support for real-time text (RTT) phone calls for the deaf and hard of hearing
Improves VoiceOver stability in Messages, Settings, App Store and Music
Resolves an issue that prevented VoiceOver from announcing incoming Notifications