In IE11, checkboxes are rendered as a flat style with a black checkmark, as apposed to the 3D look rendered by IE9. When using a RadTreeView with checkboxs and form decorator, the checkboxes are not styled to look like the 3D ones. Checkboxes that are not in trees are styled, and therefore, the checkboxes look differently. It's expecially apparent when both treed and non-tree checkboxes are on the same page. See my support ticket 815961 for my question about this.
When disabled radiobuttons are decorated by RafFormDecorator they are rendered as enabled in Internet Explorer 11. The issue could be worked around by setting "RadioButtons" to the ControlsToSkip attribute of the FormDecorator.
Currently decorated inputs by RadFormDecorator are set with TabIndex="-1", so that they cannot be focused. Expose an additional property in RadFormDecorator through which the decorated button can be focused. For the time being the focus() can be set to the parentNode of the button. For example: document.getElementById('Button1').parentNode.focus();