Rick Schaut of Microsoft's Mac Business Unit explains what went wrong with version 6 of Word for Mac:

In order to understand why Mac Word 6.0 was a crappy product, we need to understand both the historical background that led to some key decisions, and we need to understand some of the technical problems that resulted from those decisions...

Having reaped the benefits of a decade’s worth of Moore’s Law, we who now think very little of putting 128 MB or even a half a GB of memory into a laptop computer might find it difficult to grasp just how much of a problem the 68K memory wall presented for Mac Word 6.0.  But we were trying to get the whole thing to run in 4 MB of memory—that’s total system memory, not just the application partition.

Bonus link: Schaut on Carbon versus Cocoa.