Some individual applications may not be compatible, particularly if they require low-level access to the operating system--as, for example, antivirus packages do. Similarly, some hardware drivers may be incompatible, though Windows 8 setup will usually (but not always) replace those with supported drivers.

In particular, you'll want to identify Windows 8 drivers for your networking hardware. I've seen Windows 8 install on a system without any visible drama, only to discover that it had failed to install a networking driver. So it's best to have all of your key drivers on a USB memory stick, ready to go if you need them. Most manufacturers have already begun putting up Windows 8 drivers, so you should start poking around in their online support pages.

Another issue is your system BIOS.

Windows 8 requires a feature called data execution prevention (DEP). DEP prevents certain types of misbehaving code from running. Specifically, it prevents code running from within sections of memory marked as data pages. Such code can originate from badly written applications, but more often it's a sign of malware. Windows 8 needs to have DEP enabled in the BIOS, and some older systems may have a BIOS that lacks DEP support, in which case you may need a BIOS update to enable it. Socket 478 Pentium 4 and earlier systems may not have the capability at all--in which case, Windows 8 won't run, even if it installs without complaint.

Even if your BIOS is up to snuff, you may run into hardware or software incompatibilities. Windows 8 is supposed to work with any hardware that runs on Windows 7, but older hardware may still be problematic. And software that requires driver-level access may have issues, too.