For the summer 2016 release on September 23rd, there are three developer-oriented features being released.
Email 2.0 Support in the REST API
All pre-existing Asset APIs which were only compatible with v1.0 Emails and Templates are now enabled for use with v2.0 email assets.
Push Lead to Marketo
Push Lead is an alternate lead synchronization method designed for easier triggering in Smart Campaigns. You can create a single activity log item, associate a lead, and update the lead record in one call. This works similarly to a single form fill out by a lead, and can more easily be used as a proxy method for form submission in lieu of using the existing Sync Leads method.
HTTP Compression
The REST API can now compress responses using the standard defined by HTTP 1.1 specification. This can help reduce the size of the response which will increase transfer speed, and minimize bandwidth utilization.