I love the visitor pattern, it enables open/closed principle which is a great fundamental part of maintainability and clean code. You can read more about the pattern here. There is one down side of this pattern, and that is navigation. Consider this code.
If you navigate to ExecuteCommand you will just end up at some close to the metal code that executes your command handlers. And if you try to find all usages for the Command you will only find usages of its constructor (becasuse of the new keyword).
With vanilla ReSharper you need to first navigate to the class and then do a find all usages on the class declaration and navigate to the command handler from there. Very counter productive. But ReSharper is extendable!