Custom dialogs that automatically resize, or perhaps have the ability to scroll the content, if it is bigger than the set size.
I'm not sure if it's the same on a Mac, but definitely on Windows.
This would stop the possibility of the end of messages being missed if the users dialog window is set to a small size.
It seems more and more we're moving away from custom dialogs, using instead popovers, sliders or modal windows to provide dynamic messages and data-input fields when we need to interact with the User.
I guess it would be nice for those that still use them though. :-)