Among the hallmark features are the addition of Apple Pay support for iPhone 6 and iPhone 6 Plus, along with new features and functionality like MMS/SMS Handoff support on supported Macs. There's a lot of other stuff besides. Here's the complete list, according to Apple's release notes:
· Apple Pay support for iPhone 6 and iPhone 6 Plus (U.S. only)
· Photos includes new features, improvements and fixes
· Adds iCloud Photo Library as a beta service
· Adds Camera Roll album in Photos app and My Photo Stream album when iCloud Photo
Library is not enabled.
· Provides alerts when running low on space before capturing Time Lapse videos
· Messages includes new features, improvements and fixes
· Adds the ability for iPhone users to send and receive SMS and MMS text messages from
their iPad and Mac
· Resolves an issue where search would sometimes not display results
· Fixes a bug that caused read messages to not be marked as read
· Fixes issues with group messaging
· Resolves issues with Wi-Fi performance that could occur when connected to some base
stations
· Fixes an issue that could prevent connections to Bluetooth hands-free devices
· Fixes bugs that could cause screen rotation to stop working
· Adds an option to select between 2G, 3G or LTE networks for cellular data
· Fixes an issue in Safari where videos would sometimes not play
· Adds AirDrop support for Passbook passes
· Adds an option to enable Dictation in Settings for Keyboards, separate from Siri
· Enables HealthKit apps to access data in the background
· Accessibility improvements and fixes
· Fixes an issue that prevented Guided Access from working properly
· Fixes a bug where VoiceOver would not work with 3rd party keyboards
· Improves stability and audio quality when using MFi Hearing Aids with iPhone 6 and iPhone
6 Plus
· Fixes an issue with VoiceOver where tone dialing would get stuck on a tone until dialing
another number
· Improves reliability when using handwriting. Bluetooth keyboards and Braille displays with
VoiceOver
· Fixes an issue that was preventing the use of OS X Caching Server for iOS updates