Different Directory Separator

MacOS's directory separator is : (colon). This makes it a bit odd out of the bunch since before that was the case, the OS had no actual directories.

Cooperating Tasking

The OS is cooperatively tasked, although there may be multi-threading.

Development

Since Mac OS 9 is quite dated, getting information from it may be difficult.

Pascal

The environment appears to be very laced with Pascal, which I really last touched a decade ago in high school. However, Pascal is quite easy.

Environments

Environments for building programs.

Macintosh Programmer's Workshop (MPW)

Libraries