Allowing to bind multiple properties to one target object.
Would require IMultiValueConverter.
<Button x:Name="NextImageButton" >
<Button.IsEnabled>
<MultiBinding Converter="{StaticResource SelectedItemIndexIsNotLastToBoolean}">
<Binding Mode="OneWay" ElementName="ImageListBox" Path="SelectedIndex" />
<Binding Mode="OneWay" ElementName="ImageListBox" Path="Items.Count" />
</MultiBinding>
</Button.IsEnabled>
<Image Source="Icons/navigate_right.png"/>
</Button>

Allow adding custom MarkupExtensions for cleaner code.

When the browser is closed allow the app to call a web service. Good for storing information in the DB and logging.

This would be hugely useful for a number of scenarios, not just fancy UI reflections. :)

Its a big roadblock for applications that provide undo redo on a number of other actions and/or multiple RichTextArea controls and need to provide application wide undo\redo.
Discussion at http://forums.silverlight.net/forums/p/162199/364466.aspx

Thanks

Make non-rectangular shapred Windows possible in out of browser mode, by supporting transparency.

Silverlight is a very dynamic platform. It is possible to store UI in the database and have it dynamically loaded. This is a very important feature but would but exponentially enhanced if the UI rules could be controlled by a Workflow

For XAML serialization, this is needed. Also, when I'm trying to figure out what string to push into XamlReader.Load, this would be helpful to get it right. I just spent several hours trying to figure out some XAML with the correct namespaces etc. and dynamic UI programming would benefit from this a lot. And, they're a pair; they belong together!

It should be possible to receive video streams from standard encoding devices other than Expression Encoder and Smooth streaming where files are streamed. A full standard compliant RTSP/RTP stack that can decode H.264/MPEG-4 would go a long way to make silverlight a viable platform for a universal video player

There is a lot of focus towards plug in architecture, and functionality loaded on the fly these (potentially coming from third parties). A problem with this is that if two different plugin need two differet version of the same assembly (say SL toolkit or Reactive Extensions) then only one version can be loaded and therefore plugins break.
Maybe we need to have full contract of app domains then..

So far, Silverlight does not feature any of the WPF-like functionality for encryption, creating/verifying signatures etc. located in System.Security.Cryptography in WPF. Having these features in SL would be ground-breaking, as it would allow us to implement better authentication mechanisms for web services, protocols like WS-Trust etc. etc...

Line of business applications require multiple pages. Users work in them all day long - memory allocations go through the roof - and performance slows to a crawl. Windbg shows me what's left in memory but after hours of effort I still can't free up memory. Removing all event handlers (xaml and asyn wcf) building weak reference wrappers around all my converters, nulling everything I can think off, removing every control from the visual tree is NOT the answer. How about a simple pgLOB.DeAllocate statement that we can call to get rid of everything assocated with a page?

Today they announced the WPF Ribbon Control was made available as an RTW. Since it is still maintained out of band of WPF, can we get a separate version maintained for Silverlight? I now do 0% WPF development, a Silverlight ribbon would be far more useful than WPF.

The posibility that save / restore includes the inlineuicontainer added content

SharedSizeGroup in SIlverlight like in WPF so we can use ItemsControl instead of DataGrid for showing read-only data.