The two biggest impediments to restoration are:
-not having enough money;
-having too much money.
As most of the postings suggest, the major issues are political, attitudinal, an the planning process. To Geoffrey Childs' comment about a Yale education, I would add that the nature of modern training (landscape architecture in the classroom) for most golf course designers leads them to overlook much of the genius and inventiveness of the classical designers. Along the way, they are used to using big bulldozers, not little machinery, handwork and imagination. They tend to be "Fountainhead" types, wanting to build, move, blast and create anew.
Mucci asks above whether lengthening is consistent wth or antithetical to restoration. Pat, the answer depends on how it's done. Adding tees can often have a restorative effect, if you keep the more forward tees for higher handicappers and just use the new back tees to bring the older features into play more in the driving area where they were supposed to be in the first place.
However, the mortal sin with lengthening is building new greens for the yardage, in which case you usually distort the features and what you build doesn't look like what you had.
There are rare times when it makes sense in the name of restoration to rebuild a green to make better use of a site or to solve a routing problem - or simply because you are sure you can create something that has retro look with the new green complex. In such cases, be crafeul, because the success of the whole proiject will depend on how this element turns out. But generally, such an approach ought to be avoided.
The major impediment structurally in restoration is not the feature itself - any mound or bunker can be rebuilt. The real issue that no one has mentioned is the topography. A well-scuplted little bunker on a rise 145 yards off the tee cannot be moved onto a low area behind it that's 195 away, since this won't function the same way and can't be seen - and probably can't even drain. So it's topography that's the limiting factor, not the feature itself.