SSMS is based on the new VS 2017 Isolated Shell. This means a modern shell that unlocks all the accessibility features from both SSMS and VS 2017.
Smaller download size (~400 MB). This is less than half of what SSMS 17.x is.
SSMS can be installed in a custom folder. Currently, this is only available on the command line setup. Pass the extra argument to SSMS-Setup-ENU.exe, SSMSInstallRoot = C:\MyFolder
High DPI enabled by default.
Better support for multiple monitors to ensure dialogs and windows pop up on the expected monitor.
Isolation from SQL Engine. SSMS does not share components with SQL engine anymore. More isolation from SQL engine allows for more frequent updates.
Package Ids no longer needed to develop SSMS Extensions.
18.0 will install alongside 17, so you can have both at the same time.