Architecturally .NET is very much similar to JAVA, except that the core framework only runs on windows or other Microsoft supported platforms. However the .NET framework and Application Server (IIS) is not open. One will not have the flexibility to add custom modules as easily as in case of LAMP, JAVA, or MONO. Most consumers and developers find this monopolistic and are always cautious while choosing this framework.
After nearly 3 years of industry experience in Microsoft technologies, if asked which platform I would personally support for web-application development, I will blindly say PHP.
Some of the reasons
There are different statistics drawn from different companies, but among the developer world, there is wide acceptance that C++ is possibly the most widely used programming language. C++ has greatly influenced many other popular programming languages, most notably C# and Java.
MONO is an open community platform; that brings the power of C# specification defined by ECMA and Microsoft to all POSIX platforms as well as Windows. When it was released, it only allowed for Console and Web application development. Since GDI based forms implementation would not work on other Operating systems, Desktop application development was not present in earlier versions of MONO. However, recently the community created GTK# (GUI Took Kit Sharp) which is a C# implementation of GTK+ (in C++). So now, MONO developers are equally capable of creating Console, Desktop and Client-Server applications.