FieldControls that use 'selectors' (the ImageField is a classic case) provide their 'selector' window as an iframe that is embedded into the parent dialog window.
If you are creating a complex designer, this introduces all sorts of issues, as the selector dialog is thus embedded into the base dialog of the designer, resulting in a merging of parent and child dialogs that usually ends up as a real mess.
Sure, you can screw about with resizing and hiding of elements, but it's impossible to get it to look right, as there is always a conflict between them. I always end up with selector dialogs that are a hassle for me to use, and very confusing for less familiar users than me.
What I would like is an option to open the selector dialog in a separate rad-window, and have the FieldControl manage all the issues of updating from the return value - much like this example on the aspnet-ajax controls demo site.
I would also like to have events that I could subscribe for all selectors, so that I know when the selector dialog is being opened and closed, allowing me to make 'intelligent' decisions on what actions I might want to take. At the moment the ImageField has no such events, forcing you to 'fake' it with multiple element events, and it's most unsatisfactory.