As spotted by Paul Thurrott, Microsoft briefly published a document that enumerated the major differences between Windows 10 for ARM processors and Windows 10 for x86 chips. Though the document has now been removed, a cached copy is still available.
Many of the differences are predictable consequences of the different architecture. Windows 10 for ARM is a 64-bit ARM operating system. It can natively run both 32-bit and 64-bit ARM applications (though the SDK for the latter is currently, and temporarily, incomplete). As such, drivers for the operating system need to be 64-bit ARM drivers; existing 32- and 64-bit x86 drivers won't work.
This isn't a surprise; 64-bit x86 Windows can't use 32-bit drivers, either, even though 64-bit Windows can generally run 32-bit applications without even requiring any kind of emulation. This will mean that ARM Windows has limited hardware support relative to x86. It will also pose a problem for some games that use drivers for their copy protection.