I thought that this thread was about visuals and not other features, but ok. First of all, Yosemite does fix a number of bugs and also introduces substantial optimisations over Mavericks, at least in my experience. Overall UI responsiveness, Mail, Safari, Calendar are quicker. I had some issues with WiFi and Mail on Mavericks, which have been fixed with Yosemite.
However, as you are asking about progress here are just few things that quickly come to my mind. Yosemite's very substantial innovation is its introduction of a generalised plugin API. With it, developers can ship plugins that integrate in different kinds of other apps seamlessly. Another feature is OS-wide scriptability with JavaScript, which has also been introduced in Yosemite. Not to mention the whole iOS integration features. There are tons of new APIs and little improvements that makes it easier for developer to create good software faster.
But obviously, these things do not have anything to do with looks. So the discussion these kinds of improvements is orthogonal to the discussion of the OS design.
I wasn't talking about new features or bug fixes I was talking explicitly about the UI. There are many posts in this thread that refer to those who dislike Yosemite as unwilling to accept progress. It has nothing to do with the changes you've listed, which could have been done using Mavericks. Might i add that if they didn't decide to beat the UI to pulp, and Yosemite had the Mavericks UI with the new modifications/enhancements, the OS would probably already be released by now.
I suppose the marketing and sales geniuses behind this move are sitting back trying to calculate just exactly how many users they'll lose by making this move as opposed to how many they think they'll convert from Windows to Yosemite.