Under the hood is some substantial stuff.
This is a very subjective topic...
In reality, Lion was an awful update under the hood, and ML did little to remedy it's performance shortcomings.
It was not at all like Leopard to Snow Leopard.
I'm viewing Mavericks as being more what ML should have been. Somewhere over the last 3 releases, I can see people thinking they are due a freebie even if they're dreaming.
I think we all need to change our thinking. An os update used to be $129 affair, and for 2-3 years, updates to it were often major with some minor fixes in between.
Now, we do not see any major updates to an os cycle, just minor bug fixes. (And usually not fixes for major issues.... those are lucky to come to the next major release.)
The last 3 or 4 versions of OSX have been glorified service packs (and sorry fan boys, the majority of people agree on this, quit trying to defend apple, this is reality.)
So basically, what we have today is more like OSX installment plans.... years ago, you'd pay $129, get at least 2 major updates and many minor bug fix updates X.X.1 - X.X.8 Those numbers might climb, but an updates size is tiny and it's changes few. No killer new features added off cycle, no major changes, etc.
If you take Lion, ML, and Mavericks and combine what you paid for all 3, factor 3 years of time, you get something close to what you would have paid for OSX when it was a longer time between releases.
It's really just.... you're getting installment plans on the os.
So should it be free? No. And in the shorter release cycle we now have, it does allow a few nice changes that would not have been feasible as point updates, though minor.
Now lets get to something important, like can they ever fix this fracking wake from sleep thing? For the love of god, shutting down and cold starting would be faster. I don't live in Europe. I don't need their stupid power requirements on my machine.