NoSQL and RDBMS (how to make a decision)

Using the right tool for the job is the best advice I have heard on this subject but how do you know what the right tool is if you dont have years of hard won experience? What type of information do you need to know to make an informed decision? Lets have a discussion that will be relevant to 2012 problems and available technologies, but lets also discuss history so that there is an understanding of how we got here.