AEM 6.4 features which a developer must aware of

AEM 6.4 has a lot more features that directly enable the developer's activities. Below given my first observation which may help a developer in his day-to-day activities while working with AEM 6.4. This list may not be complete but definitely gives a good idea about the new things.

There are cloud-related upgrades, Fluid experience support, efficient utilization of content intelligence, etc in 6.4

Keynotes on new features:
Core module version changes

AEM 6.4 builds on top of updated versions of the OSGi-based framework (Apache Sling and Apache Felix)
Java Content Repository has the latest version: Apache Jackrabbit Oak 1.8.2
The Quickstart uses Eclipse Jetty 9.3.22 as a Servlet engine.

A new Style System is available which allows users in Page Editor to select from a subset of styles via UI. The Core Components which were part of AEM 6.3, and are now joined by the Style System, provide an efficient way to build new and extend existing sites.
The workflow model editor is now more user-friendly.
Assets improvements related to machine learning like smart crop, smart tag, smart search, etc reduce the manual activities drastically.

Some of the location/path changes to be noted are,
Template design information location is moved from /etc/design to support sites specific settings in /conf. But clients who are moving from an older version to a new version can remain using /etc/design.
Moved default location for segments into /conf to have site/context-specific Segment lists.

Component & Template Development related upgrades
Project Archetype 13+
HTL version 1.3.1
Core Components 2.0.4

There are a set of UI changes too.
'Content Tree' rail to quickly navigate a hierarchy
Improved interaction with the search results
Keyboard shortcuts, for most common actions
Ability to enable/disable keyboard shortcuts

Content Fragments & Services, Experience Fragments
Introduced Structured Content Models as a foundation for content fragments.
New Model editor UI is available with Pre-configured data elements for content fragment models.
Improved the usability of the AEM Content Fragment editor like Enhanced rich text editing capabilities, Full-screen editing for single elements.
Content Service is enhanced with JSON output via Sling Model Exporter.
Integrated AEM Experience Fragments with Adobe Target.