The Classic Mac OS refers to the Macintosh OS prior to version 10. It had several flaws, the biggest being a lack of memory protection and its usage of CooperativeThreading. It wasn't possible to fix these flaws without breaking the system entirely because it was originally written for machines that lacked a MemoryManagementUnit. MacOsxOs replaces The Classic Mac OS and is much improved.
Additionally, it is generally agreed that this Operating System is extremely ugly, and difficult on the user, and, in many ways, it always has been. The interface is often percieved as "dull", with simple, almost Windows-like icons and graphics, and the ever-present problem of windows that cannot be "minimised" without closing them completely, though later versions of the Operating System allowed the user to minimise the window to a single strip that floated annoyingly whereever the window was being it was clicked.