Microsoft offers up its view on what makes for a good gadget in a document buried under their Windows Vista User Experience Guidelines: Windows Sidebar Gadgets.
Despite being hidden away on the MSDN site, this is actually a really focused, useful set of guidelines for anyone thinking about creating desktop gadgets.
After some basic info on what a gadget is, the document explores what type of functionality belongs in a desktop gadget. There are visuals comparing examples of good and bad gadget layouts. There’s also guidelines for making the most of the Sidebar gadget framework, advice on handling different states of interaction visually, and recommended sizing standards.