I'm not sure I'd agree with Mavericks being a "bust." I've been very happy with it, and so has everyone I know who has used it.
I'm not happy with it. It got rid of labels, which we use at work to make instant decisions about which ads are pickups, new or house ads.
And it wasn't until we got our new Windows 2012 Server that I realized the networking bug Mavericks has in it's implementation of SMB2. Nothing says irritation like trying to work on an InDesign file on the server that force closes InDesign every time you try to save. The only solution is to either connect via CIFS which forces the old SMB1 protocol (slower file transfers) or to disconnect from the server altogether and then reconnect. A real PITA when your have to reset your folders because Mavericks never remembers how you set them.
As to Lion and Mountail Lion, one of the more annoying things I hate that they introduced is the absolute minimum folder size you can make your folders and the fact that when you resize them it always draws the name column in with it. So, once you've resized the folder you have to move the column divider over because your folder and filenames now show elipses and not the entire name!
Honestly. Try and size a folder down to as small as you can get it using Lion, ML, Mav or Yosemite. You can't. So, now I have giant size folders taking up screen space on three monitors! Under Leopard I can make them much smaller.
Yosemite fixed the networking bug, but I still don't have labels and I cannot see those stupid colored tag balls. Also, I don't use Mail, so nothing ever shows up in the NC for me. I use Outlook 2011.
Feature wise, I actually like some of the changes. In particular, I like the fact that when working with multiple monitors, I can cycle between virtual desktops independently on the monitors in Mission Control. A lot of people hated that Mission Control replaced Spaces(and after using Spaces in 10.5 I can understand the preference) but IMO the independent cycling makes up for a LOT of the shortcomings of Mission Control.
I thought at first that I wouldn't like the message center and other iOS like features, but I have to say that I've really come to appreciate them. When an email or text comes in, the pop-up in the upper right hand corner lets me see if it's something I need to deal with immediately or if it can wait(and particularly with email, if I can trash it immediately) rather than having to grab my phone or switch desktops/use expose to open Mail or Messages.
I've had zero stability issues with Mavericks, and my impression is that-as a whole-it's a very solid OS.
Yosemite is a different story-I still can't get use to the "look" despite the fact that I've been using it on my Macbook(secondary computer) since Public Beta 2. I did download the default Tiger wallpaper and felt a lot more comfortable on it, as the dock is very Tiger like.
Not with you on the Spaces thing. Took a while to adjust and I still don't care for it.
Mav was stable for us, until Windows Server 2012 came along and showed us the bug in Mav's SMB2 implementation. Still hate tags.
About the only good thing I can say about Yosemite is that it fixed the networking bug.