Visually, I like the changes in 10.10, but it doesn’t quite feel ‘done’ yet (perhaps that’s a little unfair seeing as this is still very much a beta). The visual cues here will feel familiar to anybody that’s used iOS7 on mobile devices, iTunes now shares the ‘red’ colour scheme of the Music app on iOS for example. Several of the textured backgrounds from Mavericks have been replaced by solid, flat colours (mostly black - the new ‘turning on’ screen is gorgeous). There are also now areas where transparency and gaussian blurs are used in the design for certain apps.
Functionally, this is still very much in beta, I experienced a lot of crashing, particularly in Safari and in the Apple preinstalled software (third party apps seemed to work fine). One major difference in Yosemite to Mavericks appears in the function of the three ‘traffic light’ buttons - the green button now gives an app it’s own screen, rather than having that as a separate button to the right as it has been since Lion - this makes window management tools like Moom more imperative than before for your workflow.
As it stands, I’ve gone back to Mavericks for the time being as it’s more reliable for getting work done, but I do think that it will be a very important upgrade for OS X as a whole when it launches properly later this year. I’m quite excited to see the ‘hand off’ functions in action when iOS8 is released - particularly being able to make phone calls and send SMS from the computer if phone is on charge in another room.