As others have said here, OS X is not sufficiently well-supported by outside vendors to make the switch. My specific issues include poor driver support for large format photo-quality printers (Canon S9000 in specific, but also Epson), and high resolution film scanners (Canon and Nikon as well as some SCSI cards). These issues remain, even in Classic mode.
In addition, the cost of upgrades to a marginally improved OS X compatible applications such as Photoshop 7 and a buggy Filemaker Pro are a drawback.
Finally, up 'till now, OS 10 performance in several necessary (and, in some cases, hated) Mac applications has been reported as poor - e.g., Virtual PC. It is not clear that OS 10.2 will provide a solution for these issues as the areas in which speed improvements have been made do not seem to attack the fundamental problems with some of these applications.
In summary, while I applaud the technology and recognize the need to move to a modern OS, the reasons to move have not outweighed the reasons to stay with OS 9.2.2., so far.