Going forward, version phantom@3 will only support Node v5 and above. This adds the extra benefit of less code and faster performance.
Version 2.0.x is not backward compatible with previous versions. Most notability, method calls do not take a callback function anymore. Since node supports Promise, each of the methods return a promise. Instead of writing page.open(url, function(){}) you would have to write page.open(url).then(function(){}).
The API is much more consistent now. All properties can be read with page.property(key) and settings can be read with page.setting(key). See below for more example.