Mountain Lion wasn't awful, but maybe that's just by comparison to Lion. But I agree, I think Mavericks will be another one of those OS's that hang around for a long time like Snow Leopard before it. Snow Leopard was the most polished OS X up to that point, and Mavericks was the most polished OS X post-SL. Both focused on a lot of internal changes instead of a lot of flashy features. Both releases were about making the best OS Apple possibly could, technologically.
I really think, either with this next computer OS or the one after that, Apple is going to make some radical shifts, like they did from OS 9 to OS X. They're going to want to rethink their assumptions about what a modern operating system needs to look like, and build something new from the ground up. Think about what computers were like 15 years ago. People had one computer, not a computer and a bunch of internet connected mobile devices. There was no "cloud". Nobody was using SSDs and HDDs were tiny. Processors were 32 bit and a fraction of the speed, not to mention Macs were all PPC. Internet speeds were much too slow to stream HD video or download large files. Pixels were huge.