A few of the bigger changes include more support for vector tiles and the additional of Arcade expression language.
Vector Tiles are now supported in all ArcGIS-supported spatial references. This addition will help more of us get starting using the vector tile approach as it seems to be the future of basemaps. One downside - you still can’t print the vector tiles basemaps.
Arcade is Esri’s new, lightweight, and secure expression language. Arcade isn’t just limited to JSAPI, and will also work with ArcGIS Pro, ArcGIS Online, and the ArcGIS Runtime APIs. For now, the focus is on expressions for visualisation and labelling but it also allows for other expressions and geometry functions. The initial release only includes basic geometry functions (create and reference). In future releases, other geometry functions should be added to help calculate areas and lengths, and perform simple overlay operations within an expression - think Arcade as a light weight ArcPy Geometry object.
Geo-location has been enhanced so you don’t get caught in the browser wars with HTTP. To helps developers still using HTTP, the JSAPI no longer displays the Locate Button for non-secure web apps in any web browser. Most of the recent browser updates now block the Geo-location API for HTTP, so this update will help keep us all away from making an app that isn’t all-browser friendly.
While on the browser topic, the list of supported browsers was also changed to state “IE9+” instead of “IE7”. If you still need IE7 this is the least of your worries.