There are two kinds of using directives: using-namespace directives and using-alias directives. A using-namespace directive like:

using System;

means that any of the classes of the System namespace can be used in the subsequent code without qualification. It is similar to Java's:

import java.util.*;

If you have a lot of these kind of using directives, then, when you look at the code that follows, it is difficult to detect from which namespace a class belongs.

An alternative approach is to use a using-alias directive to give an alias for a class:

using Console = System.Console;
using Thread  = System.Threading.Thread;
...
Console.WriteLine("Hello world");
Thread.Sleep(3000);

Using this approach, you would need one using directive for each class that is used in the code.