Descriptivism describes the position that Wikipedia policy pages exist to describe existing policy, not to shape policy. In the descriptivist view of things, the way to change policy is to try something different and see if people object. This can lead to policy pages appearing more like a list of possible consequences than a directive on what to do. This fact doesn't bother descriptivists, who believe that individual users and administrators can be trusted to make decisions on a case by cases basis given all the information.
For descriptivism to work, the same skills necessary to edit Wikipedia in general are needed - a willingness to be bold and try to implement sweeping changes, a willingness to compromise with those who disagree with you, and a willingness to back down in the event that you are widely viewed as being wrong. Descriptivism, in one sense, seeks to apply the Wiki method to social policy as well as to articles - the best way to change something is to simply change it, and see if people like the change.
Descriptivism can be criticized for amounting to having no rules at all. After all, if the rules can be changed simply by not following them, they're hardly rules. It can also be criticized for promoting instruction creep as every possible permutation of results is added to policy pages. Descriptivists respond only by reminding people of the importance of common sense.
Descriptivists are generally opposed to rules-lawyering and to their opposite, proscriptivists. They are also prone to believing that polls are evil because they represent attempts to shape rather than acknowledge policy.