Better troubleshoting and diagnostic tools,
ability to trace client code, pass exceptions from server to client, etc..

Especially the ability to retrieve not only virtual key number, but mapped character.

I understand we have rotation transforms in X, Y and Z, but you cannot have text written in an Arc shape. By this i don't mean rotating each character to form an arc. I mean bending the whole text so that font is bent too. It would also be great to apply the same bent transform to items control, so list will be distorted to desired shapes: Arc, Wave, etc.

Currently Silverlight 3 is a good platform for 2D casual games but lacks the control of, say, XNA. Porting the 2D XNA API to Silverlight would make it a much better platform for game development (as a separate library). This is definitely possible (see SilverSprite on CodePlex), but official Microsoft support would be much better.

If you want application to have "import from Excel" feature, that file needs to be parsed to map columns with your database. It would be nice to do this on the client without a need to upload it to the server first. This is especially useful for large files.

There is need to access some local hardware devices. Of interest is the class of devices are mostly exposed via usb, or bluetooth to provide Location (GPS/comppass), webcam and audio data streams. Access is mostly for the purpose of reading a data stream, but some support needed for sending commands to a device. Understandably for security/privacy; a user would need to grant access for each use of a device.

to have local cahed data and syncronyze it via Sync Framework

It would be really nice if I didn't have to implement an event handler if all I wanted to do was change the visibility of an element.

Currently you cannot add things like PathGeometries as resources which forces you to create some really unmanageable xaml in certain cases.

It is like HELL to present any kind of TEXT in SL. Most of the text/documents are prepared in Word. And there is no easy way to convert those documents to xaml to show them in SL.

Add full ado.net

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

To support saving objects and states of objects better in Isolated storage without having to write cuttom code for all objects to store them in IS. Preferably Binary Serilization.

More networking options are better. Particularly with gaming.

.Net 4 includes PLINQ and TPL. Silverlight 4 should, too. I want to use these in my app and run it on both .Net 4 and Silverlight 4.