What is the most important or awaited feature in Visual Studio 2010??
The Historical Debugger. This records what happens during a run, and lets you step backwards and forwards through time in the debugger.
The new text editor written in WPF is very interesting. There is also a new extensibility framework that will make writing add-on for VS much easier (in my opinion the most important new feature). The tools for SharePoint development are nice additions. There is a new debugger that records the debugging process and has greater support for parallel programming.
The dynamic language runtime will lead to much more readable code when dealing with COM application and will be interesting to see where it takes staticaly typed languages.
As far as the IDE goes, proper, well-thought-out multi-monitor support is a big one..
Afair, they promised to include WiX 3.0. Although not perfect tool (built on top of ugly MSI plus some xml hell) but we don't have alternatives (at least from Microsoft), do we (tried their Setup projects)?
Architectural View(Designer) of the Application This enables the new developers to understand the application flow more quickly.