Typically the application blend is working on will be source controlled, as such merging the XAML will happen at some point. Having an option for each attribute to be on a seperate line (as Visual studio has) would make the merging alot easier and intuitive.

Hello?! No Motion Path Animation in Blend or Silverlight?

Although it is possible to extend Blend, I would like to see a more open and better documented plugin architecture. This way communities and third parties can add new features more easily.

When editing a User Control code behind, it would be beneficial to be able to have full support of code completion instead of having to switch back to Visual Studio 2008. Perhaps this is in VS2010 and Blend will never be the only environment needed to get all the work done?

This option is availabel in Visual Studio and it would be great to see it added to Blend. When checked this will auto load the new file when changes are made between the two tools.

Guide lines are useful when aligning elements. Guide Layers can be useful as a place to hold concept jpegs that will then be hidden during testing and not compiled in Release Mode.

Expression Dark and Expression Light are the only two themes for Blend. There either needs to be a way that is documented to add custom themes to Blend or to select from more options. Also, there should be a Quick Key for switching between themes. A common use case to switch to Expression Light would be to see a drop shadow effect. There currently is no very quick way to switch back and forth between themes.

Support PageFunction out-of-box.

Swift3D or any core business adobe-killer-plugin

It is time to give the WPF/Silverlight Pixel Shader Effect Libraries support within Blend by adding them into the System.Windows.Media.Effects namespace. All we have is Drop Shadow and Blur. It would be nice to see a full list of effects similar to PhotoShop filters.

Provide the ability to plug in custom sample data providers.

Quite often I need to quickly prototype, trying out an idea in Blend or simply import a psd/ai file to xaml. But that requires me to start a new project etc... Creating a single page would make life a lot easier (something like Kaxaml has).

It would be nice to be able to select an element on the Artboard, hit Ctrl+C and then in XAML view hit Ctrl+V and paste the XAML representing the visual into place.

When there are multiple instances of Blend open, I would like for any floating (undocked) panels to be hidden when a particular instance is not in focus. This would prevent confusion over which project/assets/resources window i am looking at, for which project.

Tab and F4 keys overlap. Why not use F4 for showing properties (like in Visual Studio)?

It can be difficult to manage a large list of Resources. Additionally functionality here would be very useful.

Add the ability to use tracking & kerning in typography

Support for expanding and collapsing XAML nodes in the Blend code editor (Similar to Visual Studio).

SIlverlight 4 is great for games. Perhaps Silverlight 5 will support 3D and XNA libraries. But all that lack of a good game engine.
I want Blend 4 or 5 to have full game engine features, even in a separate package or upgrade/version. Full 3D models, Physics, and tools, like some new hot stuff for games of Flash CS5.
Like Unity 3D and Unity plugin.

I think to speak on behalf of all browser game development.
Please Microsoft!