I have never particularly liked how fullscreen apps work on OS X. The whole concept just feels too tacked on, too full of uneasy edge cases and limitations. It has got better though, and El capitan's new mode where two apps share a full screen is good enough that I have actually found good uses for it. That does not mean the annoyances have gone away though, it is more like the utility of the mode is slowly catching up to the annoyances. They are two, really, one small and one much larger.
The smaller one is the annoyance of the menu bar appearing only when the cursor is at the top of the screen. It feels too hard to get to when needed, and too easy to activate - making things slide around a bit in the process - when I do not want it but am trying to accomplish something else near the top of the screen.
The larger annoyance is how apps in full screen mode interact with every other window in the system. To be specific: they do not, really, and that feels deeply strange to me. OS X is all about windows and often about getting stuff between, but full-screen thoroughly breaks that. No other windows are allowed to appear on top of full-screen windows, meaning it becomes a lot more difficult to reach everything else. I use two screens at work and found myself wondering why I could not drag a window from one to the other before I realized it was because the second screen housed two windows in split-screen mode. Mail has gone so far as to implement a special modal compose window, for use only when in full-screen. That, to me, feels like a serious sign something is wrong. Why should an app need to be completely redisigned for a situation which is basically just about getting as much as possible of the available space? Why would I as a user suddenly become less able to handle multiple windows just because one of them got slightly larger than usual?
So, what have I found that I like to do in split-screen? Web-app debugging: browser on the right, developer tools window on the left. Maximum space and not much need of the menu bar or anything else which might have decided to go away when entering the mode. It is also a setup which rarely interacts with other windows, but even here it becomes a little bit annoying that every command-tab over to my editor makes the whole desktop slide, rather than popping up the editor on top as usual.
All of this started and ended in this thought:
It is weird that split-screen multitasking on IOS has me excited.
No less so when considering how I enjoy and value the distraction-free or at least distraction-limited environment of IOS.