Rumour is that this will be a stability release for iOS 9 (presumably OS X as well?) [1]. I guess now everyone will be mad that there's no new features.
[1] http://www.macrumors.com/2015/02/09/ios-9-stability-optimiza...
Overall stability. I've never had so many system freezes and crashes than with Yosemite. It started with the upgrade progress that crashed on me and even a fresh reinstall didn't help.
On Lion, I let my machine run for serveral weeks without any problem. Now I have to reboot at least once a week because the kernel is working on something that seems like bitcoin mining from the looks of the cpu load.
Rebooting shows a black screen for 2 minutes and I often thought thank god I have a backup because my filesystem got screwed up (it didn't but apparently that's also a problem for some people).
Applications not quitting, Dock locking up, Finder unresponsive, drives not showing up in Diskutil.
WiFi not working or working extremely slow. This even happens when I'm connected via cable and I turn WiFi on. Suddenly I have a really slow network connection. The WiFi issues even occur on iOS devices (not mine, but some of my family members)
These are not minor annoyances. This stuff keeps me from working and gives me an extremely poor user experience. In fact it's the first time in years I'm looking forward to a Windows release.
And of course there are some smaller things like Removing features from applications, for example the whole scripting part of iWorks. From the hostname, it seems like I run a few thousand Macs in my network.
Data point re: Wi-Fi, though: I've seen friends experience Wi-fi issues on their Airs as of one recent update, but never have them myself on my 2011 Air (that I use heavily for coding and graphics work.)
Try going back and reading early Daring Fireball. It's all complaining about how everything is broken and Apple quality has fallen off a cliff.
I think pretty much most (all?) user-facing software nowadays is buggy and broken.
Hopefully it'll slowly improve as features settle into place, and maybe we figure out better languages and processes...