OS X Lion wasn't very well received, with many people sticking with Snow Leopard, and that was released 2 years after SL
Which is why I say 10.9 will be a let down. That said, given the leadership change and no rush to stick to this silly-assed annual release schedule I think 10.9 could be reinvigorated into something quite nice - a lot of the code rewritten from the ground up. Perhaps even a UI makeover. Impossible now IMO.
Also, 10.9 development started a long time ago - way before Apple's stock tanked so at the time they basically had every reason to believe that 10.8 is what users want. (More similarities to iOS).
Anyone expecting anything more in 10.9 than iOS based Maps and Siri is in for a disappointment.
Also, it's likely the price of the OS would be higher if it was on a longer cycle so they can make the same amount of money from upgrades.
Eh? Apple basic gives OSX away to sell more macs. The revenue they generate from this is nothing. I actually believe it's not free just to give it some kind of value to the end user.
Plus, who knows if Apple might release a large update (such as Lion) every two years, and a smaller update (such as ML) every other year just to keep it fresh? It could happen...
One could hope but I doubt it. Again, macs/OSX is not Apple's bread and butter. OSX exists only to sell more iPhones and iPads. (Or the other way around).