The toughest details are those that have to match up with someone else's work, or those done — even done well — when energy was really cheap. These details are a collection of some common — and tough — dovetails of existing work with retrofits or additions. Bear in mind that green remodeling means creating a new operating regime that is better, not worse, than the one that may well have been working just fine before. Integration of energy efficiency, moisture management, and indoor air quality is much more important and challenging in remodeling than in new construction.
Existing foundation walls (and basement floors) are sometimes wet, usually damp, and seldom dry. Rarely do they have perimeter drainage systems, capillary breaks between masonry and wood (or soil) or insulation. To raise the performance of existing foundation and basement floors, you have to make them manage moisture BETTER than they currently do because you are going to insulate and air seal them, significantly decreasing their drying potential.
It can cost 5 to 10 times more to retrofit a radon mitigation system than it does to do it during new construction. And there is no real way to predict what radon levels will be in a home, even with good soil mapping of radon. A straight chase from foundation to roof is important for passive radon systems (relying only on stack effect) and for hiding active ones (those with a depressurization fan).
For more information on radon and radon mitigation, see All About Radon.