Unit Testing

I have been writing a ton of unit tests of late, and I am constantly asking myself the question "What are good unit tests?" There are probably as many nuanced versions of this as their are developers, but when you boil it down, it comes down to the following:

When it comes to computer programmers and even computer aficionados, there are a number of things which are subject to debate, and at times, these can almost approach a religious holy war given the right individuals. Examples of some of these include "PC" vs. "Mac", "Windows" vs. "*NIX", "vi" vs. "emacs" and more. So it is of no surprise that even within testing, and specifically unit testing, there are some areas were competing  ideas can cause similar reactions.