WordPress 3.8 introduced one new action and five new filters:
Action triggered after all automatic updates have run.
Filter the debug email that can be sent following an automatic background core update.
Filter whether to notify comment authors of their comments on their own posts.
Include additional elements in the 'At a Glance' dashboard widget. This widget was previously 'Right Now'.
Filter a user contactmethod label.
A list of all new hooks can you also find in the hot list of WP Seek.