.NET is a great tool for developing fastly and easily big apps in a time-record.
Developing using .Net is no more, or no less time consuming than developing for other targets.
This applications aren't stand alone binaries... (that's a problem) or... not?
Depends on your target audience. If people are not inclined to download 2 megabytes worth of display drivers to get OpenGL compatibility, I don't see them downloading 70+ megabytes of .Net framework.
What about having an implementation of the .NET framework for MAC and for UNIX?
That would be nice. Unfortunately with the CLR tie-ins to DCOM and Win32 I don't see that happening in the foreseeable future.
Microsoft won't do it, but there are some open source projects to port the .NET application to other platforms.
No there aren't. Nor would that be legal. This ball is entirely in Microsofts court.
It's just a matter of time (see mono project, for example, working on linux and MacOs X with wx gadgets as GTK).
Projects like Mono and dotGNU are not cross-platform .Net frameworks. They are seperate .Net-like frameworks, with a similar class library.
if you want cross-platform compatibility, code in C or C++ for free with the GNU compiler.
Programming in C or C++ with the GNU compiler collection does not guarantee that your program will be cross-platform.
If you don't want to reinvent the wheel in every program, code in .NET.
Or one or more of the other thousands of available class libraries, including but not limited to your own source code collection.
Read the essay
"No Silver Bullet: Essence and Accidents of Software Engineering" for additional insigt as to why the .Net framework is not all you think it is.