What does the author mean by "great" classes?

Here’s a quote from Chapter 29 - Protocols:
> Like the array-as-print-queue example, really great classes are more general than the role they may play in any particular application.

I don’t get what the author means by the adjective “great” when applied to a class. Does he mean “big”, does he mean “important”, does he mean “well-written”? What is he trying to convey?