I think there is more than a merely aesthetic change of a new look, or simplicity - because it is a certain kind of "simplicity" - which does not make, at least for me, the life more simple. It seems to me more a politic: In the last years, Apple is trying to join iOs with Os, but it isn't very useful.
in aestehtics, for 2 reasons
a) a computer screen is not as little as a tablet or iphone. So what is ok on a iPod, like a notification in the middle of the screen, is highly obtrusive on a desktop monitor. Concrete: i have a retina with a very high resolution, I use normally 2 external monitors for my work. AN example: the spotlight in one corner is enough visible for me, in the center it bothers, and imagine the spotlight window appearing in the middle of your iMac 21' when you are just looking for a link to integrate into a text?
b) a context of work on a computer is not the same as the use context of an phone. When I use a computer, I am sitting in front of it, if I use it to work or study, I am in a context of concentration, at a desktop, not doing other stuff (except procrastinating).
If i am using a handheld device, i am dwelling with cotidianity, my concentration is more dissipated and the info on the screen has to jump into my eye. So, the bright colors of the folder icons would be great on a tiny phone screen, where you have to detect them fast. But when I am working on monitor for example at a edition, the bright colors jump into my eye and distract me. I am always tweaking the icons to less disruption, the bright new aesthetics just create that disruption.
Resumed: in a context of iOs, because of the velocity I want the info, the context of dissipation, and the size of the screen, I need the info coming to me, in the context of Os we are talking of a working atmo mostly (people who just want to be communicated are well served with a tablet, today, a mobile phone can take care of ALL that), so I prefer to go to the info myself IF NEEDED. Ok, it is great to have it ordered and clear where to find, but not have it jumping to me.
the other reasons are in usability and purposes.
There is this terribly thing of social media. In Lion, all the publicity was about sharing and social media integration. I was disappointed, i wanted info about the system getting faster, programming under the hood, etc. It came back with mavericks: a better CPU admin, less energy and more speed, better monitor admin. But now, again the only publicity so far is about design and social media integration, and just getting like iOs. If I would want something which can do iOs, I would get a tablet (anyway, I have a iPod 2 gen which isn't compatible with anything, and for there rest I use android). I got a lap for a purpose which isn't social media. So this system is for me again a step backward. ( Like FinalCut: it got lost, in X presets were the priority, but they aren't the purpose of serious work - if my priority would be work to get easy I would use iMovie, I prefer the liberty of choice and own control. So I had to leave FCP.)
I use my computer as a working machine, not as a socializing machine, I get paid (and pay that fore buying a computer) for my work and not for chatting.
So, please Apple, stop mixing up iOs and Os, I guess you want to influence the hardware buying, but get aware that we are two different markets. It is not only about leaving back power users and give priority to "light" users, today a "light" user who does not want to work with a computer DOES NOT NEED ONE because mobile devices can fulfill all his needs (chatting, surfing, mails, location), so improve your iOs for them, but don't mix it up with Desk/Laptop Os.