First of all, I'd like to say I've had nothing but incredible success with your ASP.Net Ajax controls. They have been able to handle every senario I've thrown at it. There's been at least two situations where I told myself "I wish Telerik had this made for me".
A graphical bordered panel with curved corners. Ability for us to assign via attributes or css the images used for tl/tr/top/left/right/bl/br/bottom.
An ImageButton supporting mouseout, mouseover, mousedown and mouseup. Certainly doing this on your own with just the javascript is really simple to do, but one thing I've yet to figure out; is how to maintian the graphical state even after a postback. For example, if you do a mouseover (changes image) and then click, the mouseover image is gone and reverts back to the original.
Just some ideas I think Telerik could easily include in it's suite of controls.