The problem is that each release is full of bugs, and each release breaks features like 3-finger swiping back in Finder (which sometimes works sometimes doesn't, while it worked all the time in SL), etc
The more frequently they release a new OS X, the more time we have to spend on a buggy system, and the less bugs they can fix until the next release.
I still think Leopard was the best, by the standards at that time. Back then features like Spaces, Time Machine, QuickLook and Stacks would be so awesome, they would really make your computer a lot better. Now you get small features added, but full of bugs and inconsistencies (like notifications, gestures, Mission Control, iCloud, AutoSave, Resume, etc
).
I think that Apple has gone from a very well controlled system with great solid features to a loosely looked-after system with tons of different small features, many that annoy you more than they help, while the old ones stick around with broken aspects and new bugs that never get noticed.