When Apple transitioned the Mac to Intel CPUs, we lost the ability to run Classic/OS 9 software. Now, with Snow Leopard, we no longer have support for older PowerPC hardware.
There's one more handicap that hasn't been widely advertised: Snow Leopard won't write to HFS-formatted disks, either.
The Hierarchical File System was introduced in 1985 with System 2.1 and was the Mac standard for data storage until 1998, when Mac OS 8.1 switched to HFS+. Despite the new standard, Macs continued to offer full support of the older HFS format. But no more. Though Snow Leopard still offers read-only access to HFS volumes, you will no longer be able to create new disk images and partitions or write to existing ones, including to save or update files.
See Computerworld's 
For some users, this impediment affects modern data as well. I'm a retrocomputing fan for whom HFS is the only file system that the Macintosh and the Apple IIGS natively have in common. I use an emulator to transfer files between the two environments, which requires writing to an HFS volume on the Mac. Now this is no longer an option. Maybe MacFUSE, an open source method for expanding Mac OS X's file system capabilities, can add HFS support, similarly to how it already has read-only access for ProDOS, the Apple II disk format.
Even modern computer users should be annoyed by this feature withdrawal. Upgrades should offer us more options, not fewer. We previously believed that Snow Leopard Server would support ZFS, which would soften the blow: we'd still lose access to legacy data, but at least the OS would offer an equivalent number of file systems. Since we didn't get ZFS, is there a reason to drop HFS support as well? Was the file system translator not 32-bit and thus not worth rewriting for 64?
If you haven't upgraded yet, you may want to copy any essential files from your HFS disks to an HFS+ volume, where Snow Leopard will let you read and write them, so that you can follow Apple in its transition away from supporting legacy formats.
CORRECTION This post originally stated that Snow Leopard had dropped all support for HFS. The blog has been updated to reflect that read-only access is still available.