One of the biggest issues in the day to day acceptance and usage of OS X will be the presence of native device drivers. These drivers are required for things like USB to serial adapters, printers, SCSI cards, scanners, video cameras, and other peripherals that operate on USB, SCSI, or firewire.
If native drivers are not available, the only method of using these devices will be to boot back into OS 9.1 -- they will not work in Classic using their 9.1 drivers (based on what I've been told and read, having asked the question in a few places).
To help track native device drivers, I have added a new category ("Drivers") to the links section of the site. There are (obviously) other web sites that track these kinds of things (use VersionTracker and search on "drivers," or go to Mac OS X Apps and look in the drivers section); I'm doing so here only out of personal interest - I want to have one fast and easy way to find all the drivers I will personally need in OS X final!
Since this was a rapidly-changing beta OS until very recently, I would expect that between now and March 24th we will see a number of OS X drivers announced by the major manufacturers. If you see any release announcements, or find a reference to available OS X drivers, feel free to submit a link, using "Drv-" as the lead-in to the name so that they're easily identifiable in the "What's New?" box.
I hope to have compiled a thorough list of available native drivers prior to the official launch date, as this is one of the first things users will be looking to find for their shiny new OS.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010118111051134