We had to skip over Vista completely, as a year after it was released there were still loads of applications that didn't work on it (including our VPN software, which boggles the mind as it's not exactly like Vista was a surprise).
We had a few security applications we use day-to-day that prevented us from even installing XP Service Pack 2, as the changes to the raw socket support broke them.
Were it not for Microsoft preventing newer versions of IE from being installed, I think we'd probably stay on XP forever.