Drivers.
Drivers are small pieces of software that tell your software (Mac OS X) how to talk to your hardware.
MOST drivers, for MOST hardware, run at the kernel level. For this hardware, the drivers must be Intel native. This means relying on third-party support.
HOWEVER, for the drivers provided in Mac OS X (many brand name printers, keyboards, mice, modems, etc,) Apple has done all the work of recompiling for Intel. If it works 'out of the box' on OS X/PPC, it will work out of the box in OS X/Intel.
For third-party drivers, it all depends. Printers probably won't work; but you can probably use CUPS to support printers. Scanners will be a different story. Some use 'software-level' drivers that MAY work on OS X/Intel through Rosetta. Others use kernel-level. These will need to be recompiled.
Sound cards (including external) need to run at the kernel level. They will be useless until either Apple provides native support, or the manufacturer provides Intel-native drivers.
Bluetooth devices should work flawlessly. This is because all Bluetooth devices fall into one of a few categories, with no specific 'device' drivers. The generic drivers work for all devices. (For example, all keyboards use the 'BT HID' driver, all cell phones that are OS X compatible use the 'BT Serial' for communication, 'BT Object Push' for file transfer, and 'BT Sync' for synchronization.) Those generic drivers are native because Apple has to make them native. Otherwise Bluetooth becomes useless.
But, I'll let you know for certain when my MacBook Pro arrives in mid-Feb.