No, I do not miss a particular control or class. I am thinking about principle design issues.
Search Engines
The extension for Silverlight applications is xap. The content is kind of binary. At the end of the line, no content for search engine spider.
Proposed solution: add metacontent, like the original XAML file.
Librarys
Some of the new controls are not included native in Silverlight base installation. Datagrid or Tabcontrol are in separate dll’s (e.g. System.Windows.Controls.Extended.dll). If such control’s or functionality like linq is used, project needs reference to one ore more additional DLL’s. That increase xap size between 100-200K per dll. Second issue is, that code is not cached localy. I would move library code to silverlight addin or add caching.
run local
I see no reason why xap code must run within a HTML page. As Silverlight is a host for xap it should be possible to install that host for xap in OS.