Force.com Canvas: Force.com Canvas continues to add useful features like the ability to access a Canvas app in the Chatter Publisher and Feed, the ability to add a Canvas app to the Salesforce1 App Mobile Navigation, support for using events between a Canvas app and a Visualforce page, as well as a new debugging console.
API Updates: New features have been added to SOQL, SOSL, REST API, SOAP API, Bulk API, Metadata API and the Streaming API. Additionally, we have opened up the ability to use the Data.com APIs, and added a new Place Order API to integrate Salesforce order data into any web or mobile application.
Developer Console: New features have been added that let you search through all files for specific snippets in your organization, change look and feel settings on your console, along with several user-experience additions like context menus.
Apex Code: Spring ‘14 adds new classes, methods and interfaces. You can now access reports in Apex with the Analytics API and updates have been made to Chatter in Apex.