The semantic diff is pretty cool. It will probably be pretty hard to apply that to C (cpp) and Lisp (macros, and especially the programmable readers in some Lisps). But maybe we don't actually need a solution for the general case and displaying "most" changes in a semantic way is enough.

I imagine these higher-level diffs could be especially useful for things where one doesn't have that much insight into the source code - for example, scanning for API changes between two versions of a Framework (where maybe even displaying only changes to public methods/exports would be enough). In any case, diffs on the AST level seem like a good, and quite under-explored, idea.