I haven't been putting thought into this post so I thought I could at least post the things I have been thinking about and some resources. This is dealing with the use and idea of the code construct (C# interfaces). Feel free to comment with your own questions, answers, or resources.
what makes a good interface?
can you really refactor out to an interface?
if so how good would that interface really be?
how good did your class before hand have to be?
if not then it's important to consider interfaces up front
how do you make the judgement to put in the effort of making a good interface?
are these really just problems with explicit static typing instead of dynamic typing or inferred static typing?
how do you ensure the least astonishment with interfaces?
links for research:
Nice post with some comments about the use of inheritance: