And why do I have to limit my choices of wallpapers?
The point is, it was working fine previously and it is not working right anymore...
What exactly isn't working? I've got around 14 desktops set up across two monitors all with their own wallpaper (not all dark and even if I turn dark off, the menu bar still looks fine, just medium gray instead and the dock isn't as "light" even in "light" mode with darker backgrounds now and is more transparent. The dark mode dock looks like Mountain Lion's dock and the bottom dock looks like Mountain Lion's side dock so there's consistency even if it's not 3D. Besides, it looks quite similar to all docks before Leopard's 3D dock. Some people thought that was stupid looking when it came out too). All the background images "work" fine. Yeah, the new icons aren't great, but unless you make them huge, they aren't that noticeable. Besides, most of my dock icons are 3rd party apps and you should be able to replace the icons if they really bother you. Yosemite's font (I didn't use Yosemite so I can't comment directly except on screenshots) wasn't so great, but I don't have readability issues with El Capitan's font. I don't know how bad the drop shadows were on Yosemite, but I don't have problems telling windows apart either. I do think they should have more of a difference between the windows in terms of the active vs inactive ones.
El Capitan's Mission Control is an improvement over Mavericks (you can see all your windows even if there are multiple windows per App and yet you can switch that back to Mavericks behavior if you prefer it). Dashboard still exists (I was concerned they'd ditch it altogether; I still have some widgets I like to use (iStat Pro and Sloth Cam, the latter viewing the Panama Canal web cams). Notifications new "Today" pane is nice (weather reports and instant calculator access here along with calendar events).
I reorganized my Launchpad setup with all programs in their own folder categories so it's actually easy to find "types" of programs now rather than one giant arse "Applications" folder (I did split "Games" out into its own folder long ago, but some have to be linked from there and that leaves Applications still messy looking). They really should have a tool or preference pane to make organizing faster and easier, though, so it's more usable (I organized it once before, but it broke out of the sub-folders somehow one day).
Launchpad (blurry Katy Perry in background):
I will say Safari constantly nagging me to change it to my default is (bleeping) annoying, though. It's just like Google nagging me to change to Chrome every other visit to their web site. If I wanted to change it, I would change it. How many damn times is it going to ask?
I do also hate the stoplight buttons (the classic "gem" look was so much better), but the world isn't going to end over a button design.
I don't like that it broke some programs (I had to upgrade VMWare Fusion from 5.x to 8 when 5.x was doing what I needed it to do just to play old XP games and despite their claims of speed improvements across generations, it doesn't run XP games any faster that I can tell; now if they added DirectX 11 support for Win7 and newer it would be worthwhile; not much uses DX10 over DX9 compared to DX11 that had a lot of fancy gaming support options. Pinball Arcade is night and day different with DX11, for example). Spotlight seems weird starting in the middle of the screen, but it does provide more information now (you do have to give it time to cache everything when you first upgrade). The disc icons aren't as good as the old ones, but again, the world won't end.
Other than that, I'm hard pressed to think of any other differences I've noticed offhand (I did have problems with the background screens not sticking when I got my 2nd monitor working again; oh yeah that did require me to get a new adapter; Amazon's $10 "Basics" model worked fine, though. I was using some Taiwan cheap thing, but it worked fine in Mavericks). El Capitan feels about the same as Mavericks GUI/speed wise (although XBench says certain things are noticeably slower, but I don't know if I can trust all of the tests at this point since it hasn't been updated in like 9 years!) Metal should hopefully provide a boost to some games at some point (Aspyr has said they may even update some of their older games where it could provide a good boost; I'm hoping for Borderlands 2 to get an update so I can turn up the settings higher).
Honestly, on a day-to-day basis, iOS was the bigger shock since the music player went "bright" and what not and it's hard to look at compared to the old interface for that reason. Icons there are huge and you can't change their size or appearance so ugly icons are more noticeable, etc.