First; I have a nice, long titlebar strip with left-aligned titles like I like. I can click anywhere in there and move my windows around (I know about alt+click, but I don't want to use my keyboard to move a window, and that doesn't work on my other OSes anyway.)
Second; I like having a choice in how my window decorations work. I can run Clearlooks-Phenix, or Aguelemon, or Be, or Crux, or whatever else I like. Some of the themes have window borders on the sides (which I like), some don't. With CSD, my only option is a poor OS X imitation theme. No edges to the sides of windows (awful ... I can spare 4px so that my window doesn't blend into my desktop, and so I can grip for window resizes easier, thanks), no control over titlebar text placement (I don't want to scan to the middle of the title bar to read the title. The left is a consistent location.)
Some of these WMs have special features like middle-click the maximize button to maximize it horizontally, nicer snap-to-window and snap-to-edge functionality, etc.
Third; I want my title bars free of clutter. The title bar is for the title of the window, and for moving it, and at most you'd have the minimize/maximize/close buttons on a corner. I've been using windows the same way since Windows 3.1; so about 23 years now. You may scream Luddite, but I say that if it's not broken, don't fix it. With CSDs, now I have to dodge all of these ridiculous controls like "Open", "New Tab", "Save" and "Hamburger" when I want to move a window. And all of those further reduce the available space for a title. So I guess I can forget about showing the full filepath for my files open in gedit.
Fourth; and this is a big one; consistency (eg discoverability.) All of my Xfce windows look the same, act the same, work the same. I have a title bar, a menu with all of my features, and usually an optional toolbar with all of the major features. Now with these CSDs, every single window looks entirely different. To get to menu commands, I have the added step of clicking on the hamburger first, which has a miniscule hit window. And that's assuming everyone designing these awful CSDs uses the same hamburger icon and placement. Again, we go from something we've been using for 23 years to "the trend of the week design." Because newer must mean better, right?
CSDs are basically what you get on Windows with every OS driver installation GUI. I do not want that for my desktop at all.
There's a reason this has all mostly stayed consistent since GUIs became a thing. It's not because people were lazy, unimaginative, not aware of design guidelines, or anything like that: it's because it works. And it works really, really well. A lot of us are simply tired of relearning how to use our computers every time a "modern" designer gets a hair up his ass and decides that we need to radically change all the things! The ribbons, the Metros, the CSDs, etc. So in four years, while I stick with my tried and true approach, Gnome 4 will have undoubtedly moved on to an even newer idea for how to design windows, and you'll be explaining to others why Gnome 3's CSD approach was flawed compared to $newTrendyDesign. And while you're learning to get used to where all of your operations have moved to, I'll be over here getting work done instead.
> Motivations are (to my understanding) to save vertical space
Even when I was stuck on 1024x768 CRTs back in the '90s, I can't say I've ever thought to myself, "if only I had an extra 10px of vertical screen space, my user experience would be vastly improved." The idea of crushing everything into this hot mess to save the same on my 2560x1600 panel (let alone 4K/8K panels) is just asinine.
The only time I've ever had issues with the titlebar taking up space was during the 640x480 fallback VGA modes when installing my video card drivers.
Besides, isn't the trend lately (even in Gnome) to add ridiculous amounts of excessive padding to everything, for the sake of all twelve touch screen desktop users?
> reduce window clutter
That window looks a lot more cluttered to me. I do turn off the toolbar (and status bar for that matter) on gedit 2, and I can turn off the menu (with an addon), and use keyboard shortcuts for my actions. I have no control over this CSD, whose height is twice that of a plain titlebar anyway (to accommodate button heights.)
> and put common actions at the forefront
The toolbar does the same thing, and common actions goes from 3 to 13 common actions in this case (with room to spare on the latter.) Even better, the toolbar is completely optional.
For all but the three common actions, it is now harder to do things with the new gedit. And terrifyingly, one of those now-harder actions is "move the window."