upgrade – What are some of the major differences between Java 7 and Java 8?
upgrade – What are some of the major differences between Java 7 and Java 8?
There are many new features in JDK 8, including enhancements to Collections, new security features, and even additions to the Java programming language (e.g., Lambda expressions). Your best bet is to take a look at Whats New in JDK 8 as there are too many to list cleanly here.
As for why you might not want to update immediately, here is the Compatibility Guide for JDK 8, which lists incompatibilities between JDKs 7 and 8.