Sophos recently released a paper titled Windows 8: Redmond's Safest Operating System Ever? and although it's presented as a question, the conclusion that seems to be drawn is that the answer is "yes." As it details, many of the security improvements are in the "little things" - enhancements to already-existing features such as ASLR and DEP, Defender, DirectAccess and the web browser.
It's a good, quick, high level overview of those improvements that could be useful in convincing the "higher ups" in management that an OS upgrade would be a worthwhile investment. You'll find a download link for the paper in this article: