After a lot of back and forth in the TC and the failure of ES4 the TC was focused on ES3.1. The final spec for ES3.1 is now called EcmaScript, Fifth Edition.
The most exciting thing in ES5 is that it finally standardize getters and setters. It also allows us to control the enumerability as well as whether properties should be read only etc. ES5 also adds a few other things like the array extras from Mozilla.