Mostly bug fixes. Between 11.20 SR6 and SR17, about 763 bugs have been fixed (officially).
Features and "improvements" include:
SR16: TLS 1.1 and 1.2 support
SR16: CAL Quarantine period increased to 7 days.
SR16: Support for Win10/IE11
SR13: sorting now works as documented in specific objects
SR11: better reporting of syntax errors in expressions
SR11: Direct Discovery Sub-queries
SR11: Various Server/Publisher improvements
SR10: New Pie chart color option
SR9: iOS8 Safari support
SR9: New "Continuously" trigger in Publisher
SR8: Configurable AJAX client messages
SR8: Configurable AJAX client functions
SR7: Direct Discovery enhancements
SR7: Task Performance summary
To perform an in-site SR upgrade, download and run the SR17 installer on your Server. All configs, tasks and license assignments will be preserved. Before starting, make a backup of C:\ProgramData\QlikTech. Just to be sure.