I am also a bit unhappy with some of the customizations lost... especially with appearance.
I feel that people should have a bit more control over fonts and colours, and I don't necessarily appreciate some of the "dumbed down" applications,(like iPhoto, for instance).
I will certainly not be tossing Aperture... was saddened that it's no longer going to be an upgradable application for much longer. Just updates to support new OS's. iPhoto is sadly inadequate, and I prefer Aperture and it's many plug-ins to Lightroom, far and away!
I do understand that the GUI is really meant to make the experience of OS's as visually seamless across all devices as can be, partly in needing to cater to a device-rich-knowledge-poor population of users. But I also understand that the less sophisticated rendering of objects,(like icons, menu bars etc.), makes for a leaner, faster GUI/OS. I can state honestly that my iMac runs Yosemite faster than it ran Mavericks... and has done since installing Beta version 1. And there is a smaller footprint...= more room for my photography,(I'm an art photographer), and my audiophile grade lossless music files.
Apple has always limited customization options; we've never been able to choose menu bar colours, particularly, or do too much with item text font choices. Not everybody is a "weak" user, and settings could be a bit more creatively available. Nice thing about Apple always has been that, if you screwed up, it has always been easy to fix things. And there's always Time Machine, if things went REALLY awry! 😉
Being able to hide the top menu bar, as we can hide the dock, would be lovely. Now that there is a duplicate bar on my second monitor, I find it even more annoying to be ever present.
BTW.... I prefer the "dark" option to the light version, and use "graphite" instead of blue... the lollipop colours of the buttons in "blue" just grit my teeth for me.
I keep sending feedback to Apple, asking for more customization.
Here's hoping!
Cheers!