Really, the removal of Aero is what ultimately pushed me away from Windows 8 when I saw the announcements back in June about the new flat and ugly desktop UI. I don't care that the "Metro" side looks the way it does, but the desktop side should have been left alone.
I'm probably alone in this thinking, but how something looks is very important to me. I see my UI every day, for a good amount of time. I never could stand how windows 95~2K looked (the classic theme), because it's so ugly and plain. No shading or details except a silly gradient strip across the titlebar (this theme looks especially bad in Windows 7, where the UI layout just makes it downright ugly). However, this new desktop UI? Hideous. It removed the only saving graces classic theme ever had. The bevels, any sort of gradient, and even the ability to customize the colors and size of every part of the UI. It's just flat. The desktop no longer feels like a space of depth, just crammed together flatness!
I've always really liked the way Aero looked in Windows 7. Not only that, you could really customize how transparent it was, how vibrant the color applied was, and nevermind the multitude of custom *.msstyles capable of making it even better in some ways. Windows 8 Consumer Preview took away the rounded corners and the shiny effect, but it still kept the transparency and even added a wallpaper-adapting color option. That option was really cool, and I didn't mind too much how it looked in the Consumer Preview overall. Release Preview turned the scrollbars and explorer UI flat, I didn't like that, but the rest still looked alright. But the RTM took away almost all transparency (except the taskbar; no blur effect), and took away the border shadows I liked too! They didn't stop there, they took away any details whatsoever in the borders and even the buttons. Sigh.
I tossed the RTM in a virtual machine to see if I could find a way to bring Aero back. I cannot.
I was able to, however, bring back the RP version of the Aero theme, but transparency is forever lost in the way it was handled in Windows 7 or earlier builds of Windows 8. If anyone finds a way to bring back the proper aero code (proper transparency and BLUR effect), let me know. I might actually buy Windows 8 if someone finds a way to bring back some of the CHOICE that Microsoft stole away from me. They made all the people who loved the classic theme happy, by giving everyone a very flat and plain UI. Yet they took away Aero from us who liked our interfaces looking nicer and more futuristic. Flat boxes are not modern or futuristic, no matter how much Microsoft tries to say it is. Sorry.
They could have EASILY kept the system they had going in the earlier builds of Windows 8. Aero for us who like pretty UIs, and turned the flat 'Aero Lite' theme they had in there into what they use now. They would then satisfy EVERYONE. It would not have been but maybe 8mb more of code in the system.
TL;DR: I want Aero back before I buy Windows 8. MS should never take my choice away for a nice looking UI.