One of the reasons for moving to VB.NET from VB6 may be the more advanced security models available in the .NET Framework - this is particularly relevant in the ASP.NET world where many .NET applications will need a security system. In the desktop arena, there may also be requirements for security features over and above those provided by the OS, which would be difficult to implement with VB6.
Even if you don't need any new security features, you can't avoid security in .NET, as it is "built-in".
The three main areas of security in the Framework are:
It's the last one, CAS, which probably needs most explanation for newcomers to .NET.
I will take a look at security over the next week or two on this blog… which should be useful recap for me as well :-)