Snow Leopard was the high watermark release for OS X, before focus and engineering talent migrated to iOS, and successive releases of OS X began to become cluttered with throwaway useless user interface candy and naggy GateKeeper dialogs.
YMMV, but a lot of that "useless user interface candy" makes my day noticeably better. Launchpad is silly, but Mission Control pretty much changed how I used a computer. I'd been using Macs since 10.4, but I didn't get Macs until Lion. And 10.9 made multi-monitor much much better by making Mission Control desktops per-monitor. This is the best OS X release by far. As far as Gatekeeper dialogs go, I'm curious what you…
YMMV, but Mission Control completely broke the way Spaces worked as compared to prior Mac OS X releases -- which also happens to be the way I've worked since the 90s.
I don't need full screen apps, or separate spaces per head, so from my perspective, that change was an enormous net loss.
There are work-arounds, but they're imperfect: http://totalspaces.binaryage.com/
Factor in silly stuff like notification center, and all Apple seems to have done UX-wise for me is make it harder to actually get work done.