What applications have you written with .NET? What parts of the framework have you used? What worked? What didn't? Was this developer error or a genuine problem with the .NET framework?
When did you evaluate Mono? What parts of it are behind .NET? What platform did you try it on? Did you use a current release, or an older version? Are you aware of what Mono actually is?
When will you stop hand-waving and come up with something concrete?