First off, great post. It makes a nice change from some of the other stuff on here! My thoughts on the exotic items are that there should be a mixture of ways of getting them, almost like sub categories of exotic items. For example some can only be obtained from random drops/treasure chests but maybe aren't quite as powerful as other exotic items, some can be obtained by completing specific difficult tasks and some can be crafted by using different rare items. Like you said, you don't want them to be so common that everyone has them, yet at the same time you don't want them to be so difficult to get that it puts people off.