When Dashboard was an infant, Apple provided developers with guidelines for developing effective and useful dashboard widgets. If you look at the “Top 50” Dashboard widgets on the Apple Downloads site, you will find that a good portion of the most popular widgets don't even follow Apple's guidelines. At All.
Apple's guidelines for developing Dashboard widgets are a good set of rules to follow, given that they will probably produce a better long-term widget, as opposed to a viral, short-lived DashToy. Not only do a lot of widget developers ignore best practices (make them small, unobtrusive, and avoid embedding flash, just to name a few), some widgets even require CPU usage when idle! By far, the worst category is obviously the games.
Here is a picture of all Top 50 running in Dashboard at once
Can you honestly say that you would use more than 3-5 of these on a regular basis? This isn't how Dashboard was intended to be used. And things will only get worse from here. With the release of OS X Leopard, we will see even more useless widgets, given the inclusion of an easier widget making tool provided by Apple.