WordPress version 4.4 comes with a lot of changes and features for WordPress Developers. I have made a brief list of major features in the release.Prior Knowledge of php,html,css and WordPress is required to be able to understand all the features listed below.

WordPress 4.4 Features

  • Taxonomy Terms: WP_Term class introduced. get_term() , get_term_by() functions will now return WP_Term objects.
  • Term Meta: Now metadata can be added for each term easily with new functions add_term_meta(), update_term_meta(), delete_term_meta(), and get_term_meta().
  • Comments: WP_Comment class introduced to query comments data.WP_Comment_Query class introduced to query through a comment tree.comment_form_field_comment, comment_form_after_fields and other hooks introduced in comment_form() function.These hooks are used to display comment textarea first in the form. You can also now easily customize the fields in the form.
  • Multisite / WordPress Network: WP_Network class introduced. Following new functions are introduced add_network_option(), update_network_option(), get_network_option(), and delete_network_option() to manage network options.
  • Rest API: REST API infrastructure has been finally introduced to WordPress core.
  • Responsive Images: Responsive image markup has been added to the img tag of output from the_post_thumbnail() with attribute srcset. New thumbnail size medium_large, has been added which is 768px wide by default, with no height limit. New function wp_get_attachment_image_srcset() and wp_get_attachment_image_sizes()can be used to get srcset and sizes attribute values for any image. You can use the values in img tag in your WordPress theme.
  • Twenty Sixteen: New Twenty Sixteen theme has been introduced.
  • oEmbed:

    You can now Embed WordPress posts in your WordPress 4.4 based website.Developers can change or edit the embed output code only using embed_head and embed_footer.

  • Accessibility of Admin Screen: Headings in custom admin page should be wrapped with h1 tag and subheadings with h2 tag. Dashboard widget headings should also be changed to h2. More details can be found here.
  • Metabox: You can now add metaboxes to specific custom post types very easily by using add_meta_box function. add_meta_box() now accepts array of post types with $screen parameter.

You can visit list of all changes that have been addressed (or the tickets that has been closed) for WordPress version 4.4 WordPress Trac.
You can also view more detailed and updated list at : here