Ext.create和new
Ext.create provides a higher level control for instantiation throughout. It's best practice from Ext JS 4 onwards to use it over the 'new' keyword, since it's tightly coupled with Ext.Loader which makes dependency resolution much easier. During development the target class will be automatically loaded synchronously the first time it's used, which frees you from having to remember "Ext.require-ing" it before-hand.

Besides, just like Ext.require, Ext.create accepts either a class name or an alias, which makes it extremely convenient to instantiate almost everything in Ext JS 4 library by its xtype / type without having to figure out its full class name up front, for example:

new My.ClassName({list of arguments must be known here});

参考:
http://www.sencha.com/forum/showthread.php?127671-Ext.create-vs-the-keyword-new