what is the role of mscorlib in dotNET?

Have a look at This[^]

hope it helps :)

I want to contribute to a correct answer by Simon.

Not only other assemblies depend on mscorlib; there is something else which always uses this library. In particular, this is C# language.

You can always avoid explicit use of any libraries at all. You can create a class library not using System namespace. For example, it could implement some pure numerical or string manipulation algorithms from scratch and export them in the form of public classes/structures and methods. (Why just the library? Yes, it can be even an application, but quite useless one as it could not output anything without the libraries.)

Will it use mscorlib? It will, because you need to use at least embedded types like int, uint, double, string and the like. Yes, all these identifiers are keywords of the language, but they are implemented as the aliases of System.Int32, System.UInt32, System.Double and System.String, respectively.

[EDIT]

Thanks to Simon for bringing the issue of dependency of all the types upon System.Object. This problem is not that simple. Consider the following:

int value = 13; //still, has nothing to do with System.Object. This is a primitive type using exactly 32 bits

System.Console.WriteLine("Value={0}", value); //value is boxed to a type derived from System.Object

//Why? because the signature of WriteLine is
static void WriteLine(string format, params object[] parameters); //accepts System.Object
//so primitive types are subject to boxing

See http://msdn.microsoft.com/en-us/library/yz2be5wk.aspx[^].

—SA