Having worked with Classic ASP for about 2 years now by creating a few 100 simple web forms I can't see a good reason for switching to .net; however, I'm not that versed in .net so I'm sure I could be missing a few things.
Some points that I often hear around work can be found here: http://www.packtpub.com/article/Classic-ASP (not that I share all of these thoughts, but it's a good jumping off point)
I'm very interested to hear what others have to say on this matter.
You're missing more than a few things! ASP.NET is orders of magnitudes more productive, robust, and maintainable than old-school ASP ever thought about being. Server side controls, third-party controls, master pages, forms authentication, forms validation, an OO model than encourages appropriate application partitioning, easy deployment, built-in debugging and tracing, state management.
You even have the choice of WebForms or MVC. It's not an understatement to say that you are simply out of your mind if you don't thoroughly investigate what you're missing.