The level of customization that Silverlight comes with - even when you don’t have access to the source code of the control you are using - is simply amazing. Those who experienced WinForms control customization or control centric web frameworks like ASP.NET already know how much trouble they face when customizing a 3rd party control, if there is any customization support at all. In fact people have been asking DevExpress to add RightToLeft support to their WinForms suite of controls, which is not going to happen for the same reason: it is hard to modify how a control looks without touching the other behavioral logics inside because those two are mixed together. Since in Silverlight and WPF the logic is completely kept separate from the presentation, you could twist the look and feel and make the control look how you want it to look.

A good example would be adding Hijri Shamsi calendar support to DevExpress scheduler controls. This now can be done by retemplating the whole Scheduler control and using date calculation support of my FarsiLibrary project. The best part is that unlike WinForms, the source code is not needed at all and you can apply this to the out of the box control directly.

DevExpress Scheduler control with Farsi / Hijri Shamsi calendar support

Microsoft abandoning Silverlight platform in favour of HTML 5 makes my heart ache. Will there be that level of customization support in HTML 5 applications (and controls, maybe someday) is left to be seen.