Thank you for writing.
One of the main differences between RadControls and standard controls is the theme support. We are able to provide theming support in our controls thanks to their architecture which is quite different than the standard controls. Our themes work only with RadControls and are not applicable to other controls. If you want to have a consistent look and feel in your application you will have to use only RadControls.
On a side note you can check out our new TelerikMetroTouch theme that is available part of the additions in 2012 Q3 SP1
I hope this is useful. Should you have further questions, I would be glad to help.
the Telerik team