Matticus provides a nice overview of the strengths and weaknesses of each of WoW’s healing classes in Assigning Healing Strategy – Part 1: Recognizing Class Strengths. This is important for for heal leads and for raid leads. I’ve raid healed on 2 of the healing classes, but it’s very helpful to have each healing class’ role in a raid laid out so neatly.
Life of a Nin has a very nice writeup On Comparing tank classes. He gives a quick overview of the pluses and minuses of each tanking class and where you would use each of the classes. For another perspective on the same subject, see WoWWikki’s Tank article and the articles for each tanking class.
Hopefully someone will do a series like this for DPS classes.