The following changes were made at version 2.7
Map navigation: Superpan is used on Apple trackpads. Refer to the map navigation conceptual help topic for more info. This navigation mode is enabled by default for machines that support it. The major changes are that a two-finger swipe/drag causes the map to pan instead of zoom in/out. To zoom in/out, hold shift + two-finger drag.
Feature layers: ONDEMAND mode feature layers automatically and appropriately generalize features. Refer to the Feature Layer best practices conceptual help topic, specifically the generalizing features section, for a more detailed explanation.
Tiled map services: more efficient loading of tiles from map services hosted on arcgis.com
esri.request now has the ability to upload files.
Breaking Change: The identity manager now requires SSL. Any app that uses the identity manager requires https.
Support for dynamic layers. Dynamic layers allow client applications to specify new renderers for data, add data from registered workspace and join data from other resources published with ArcGIS Server.
Geodesic buffering with a geometry service.
Feature Layers best practices. This information was previously available on the ArcGIS Server blog but has been added to the official SDK documentation.
Recommendations for dealing with large numbers of map graphics.
Additions and changes to samples:
Bug fixes for 2.7 include: