1. The first item is an understanding of HTML, XML/XSL, and CSS as a structural baseline to understand page rendering in SharePoint. Having some dynamic Web langauage experience would also be good (Such as JavaScript).
2. The second item is an understanding of ASP.NET. If you don't have skills in these first two items out of the gate, you'll certainly gain them as you develop more with SharePoint. For those who asr ASP.NET developers today, the transition to SharePoint development will be much easier.
3. The thrid item is a baseline understanding of one of the managed code languages - that is, C# or Visual Basic (VB.NET). These are object-oriented languages that fully leverage the .NET Framework, and both are supported within Visual Studio 2010 for SharePoint 2010 development.
4. The last is a wider understanding of the .NET Framework such as Windows Workflow Foundataion or WCF. Again, this is something that will come with experience, but be open to learning .NET because you'll begin to understand elements you can apply to your SharePoint development efforts.