1) .NET frameworks are installed in a well known location (%windir%\Microsoft.NET\Framework). It would be fantastic if SE could automatically tag all the framework versions on the machine and then apply the right tag files to .NET projects based on the target CLR version.
2) Better WPF/XAML/C# integration - completion lists in XAML
3) Support for .NET 4.0, C# 4.0
4) Graphical XAML previewer - I don't like the designer in VS, so I write all my XAML by hand anyway. It would be fantastic to get immediate visual feedback on my XAML in SE.
5) Better Ruby support, built-in debugger support
6) Create/modify C# projects in SE
7) Built-in support for common build systems like Ant, Nant, and msbuild. It would be pretty cool to be able to tell SE to create a "C# (msbuild)" project, add files to it in SE, and then be able to open the .csproj files in VS for debugging purposes.

Ability to open VC++ 2010 .vcxproj project files
9) Take better advantage of multi-processor systems. Do more parsing/indexing in the background so the editor can be more responsive to typing.