Anything that requires a 64-bit OS and, by extension, anything that needs more than ~3.5Gb of RAM to run (well) as well as anything that needs DX10+ and any new hardware that doesn't ship with XP drivers. This might not be that much now, but it will start to increase dramatically from today onwards.
And no, XP 64-bit does not count, it's a bastard hybrid of XP & 2003 Server and nothing really supports it properly.
If forced into it today, business just might adopt Linux and WINE to run their apps and find out they are safer and more stable because of it
No, they won't, at least not in statistically significant numbers. The cost and hassle of an XP->7 upgrade is much less than the cost and hassle of an XP->Linux upgrade due mostly to the retraining costs for both IT and users (deny it all you like but Linux is not similar enough to Windows that average users will just be able to run with it, most of them panic & phone support when one of their desktop shortcuts disappears) as well as the fact that most of the desktop hardware currently running XP is too old to run a modern Linux distro comfortably - a bunch of Celerons with 256Mb of RAM might be fine if you're running a stripped down install with XFCE and don't want to run any intensive applications, but Linux is not a magic bullet that makes old PCs run like new ones.
On top of all that, you lose integration with all the Windows-based server side systems you already have in place, which leads to either whole-sale replacement or lots of fudging things so they sort of work like before.
I honestly don't think most people advocating businesses should switch all their machines to Linux instead of upgrading from XP really appreciate just how much work would be involved in doing so. Home users are a different matter and it's much more practical for them to take that route, in theory.