- I use a desktop for a lot of tasks simultaneously - but usually one task where interruptions mean a loss of 20 min to 3 hours work, depending on how important and attention-consuming the task. This is priority #1 for me. Gnome 2 and KDE are both acceptable, as is Windows 95+ (but not 8) and MacOS. Mobile tends to be acceptable.
- I need to be able to organize files : database/file management interfaces should make sense and function. (Windows 8 does not make this easy to find, and KDE and Gnome 3 actually have the best). Mobile misses this entirely.
- Ability to organize tasks when multitasking. Task switching needs to make sense, and multiple desktops are quite handy when having to do cross referencing or coding. I am a programmer - having one desktop for browsing, another for code editing and testing and a third one for communications is pretty much my ideal basis. Classical desktops (Gnome 2 and KDE - and I find Gnome 2 slightly better) do this quite well. Newer systems have lost this. Lacking multiple desktops tends to keep me out of Windows or MacOS, for the most part - although the latter is ok at them.
- Saving and restoring of state, particularly for console sessions. A lot of what I do still takes place in terminals, particularly when managing outside servers. Gnome 3 lost this - in a fit of "bug fixing" all infrastructure was removed and there is no real support for this now. KDE is ok, MacOS is quite excellent. Android and IOS are VERY good at this and I sure wish ssh and console was more usable on those platforms.
- Recovery after failure. A number of issues can cause - with power failure coming at the top, but not excluding non-fatal hardware faults. Mobile is quite excellent, MacOS is quite excellent and KDE is ok. Without the ability to handle state, Gnome + Windows do not have this.
While I miss desktop isolated task switching and the ability to return to the last task I was working in under KDE (IMO that's a bug), this is largely why KDE is now my default desktop of choice.
I really wish I could do most of my programming tasks on mobile, but there's no infrastructure for it - they have no ability to handle multiple tasks.
I kind of miss the multiple consoles of nongraphical linux - but it's a bit hard to do things like web browsing there.
Windows 8, Unity and other such "interfaces" are not desktops and have moved away from the ability to perform routine (as in : continuous and important) tasks.