Crucible is doing a great job presenting the diffs. However, sometimes it is really necessary to launch an external diff to be able to perform a complex code review. E.g. if a lot of code has been moved around, I will want to launch gvimdiff and move the code around manually until I see only the meaningful differences.
If Crucible has a quick link to launch external diff tool on the currently viewed file revisions, it would be extremely helpful.