Not too many people seem to have noticed that something rather momentous just happened with El Capitan. For the first time ever in the history of Mac OS (as far as I know), there is an official option to automatically hide and show the menu bar, just like you can auto-hide the Dock. Look in System Preferences -> General. This option is definitely not present in Yosemite and is something I tried to do in various ways for several years after switching to OS X from a combination of Windows and Linux over a decade ago. There was an Unsanity APE haxie years ago called Menufela that was able to hide the menu bar, but it stopped working around the time Tiger came out. I had given up on ever achieving my dreams of minimizing unnecessary stuff being shown on my screen in OS X, but now it is possible. I even tried something called MenuShade for a while, that would overlay a black bar on the menu bar to "hide" it, but after a while that seemed sort of pointless.
I can now hide both the menu bar and the Dock and stretch my windows (if I so choose) to fill all available physical screen space. The menu bar and Dock will pop up and overlap the window, then disappear. This is fabulous on a small screen, but very nice even on a 17" MacBook Pro screen or a 27" iMac. You might think, oh, just go fullscreen, but you'd be missing the point. Fullscreen apps outside of the tablet/smartphone context are mostly a joke to me, because there is a only a single app (Screen Sharing) that I ever really want to use on the screen all by itself. It's a bad paradigm for most apps, as evidenced by the fact that Apple are now shoehorning a tabbed multi-window interface back into the fullscreen mode of Apple Mail, because obviously people found a single overlayed compose window in fullscreen Mail to be too constricting in practical usage. Fullscreen on the desktop is pointless except for things like gaming or Screen Sharing where you really only want to see that one task on screen without any distractions. Even a split-screen view with two fullscreen apps is just not that great. It forces you into a completely different way of interacting with the windows on the screen.
I frequently just want to work with a single window or a couple of different windows in different apps at the same time, but minimize visual distractions on the screen without necessarily filling the screen with the app window. To help me with that, I have an app called Isolator that will overlay any color you choose at any opacity level on everything behind the application that's in focus. It's pretty old but still works with Mavericks, Yosemite and El Capitan. Just google "download isolator willmore" to find it. The remaining thing that helps me size windows the way I want is the SizeWell SIMBL plugin, which amazingly still works with Mavericks, Yosemite and El Capitan also.
But really the final piece of the puzzle was being able to hide the menu bar, and now that is possible without any hacks. These tools, plus the four-finger swipe up and down on the trackpad for Exposé functions, are transforming the way I'm using my MacBook Pro, and this is a computer I've been using daily for several years, so that's really saying something.
From my personal perspective, the ability to auto-hide the menu bar is one of the defining features of El Capitan, and it was never even demonstrated or mentioned anywhere. I ended up stumbling upon it accidentally while trying to remember where to set "Reduce Transparency". It was like opening my sock drawer one sleepy morning and suddenly finding that while I was sleeping someone had lined it with 24 karat gold foil and filled it with live puppies, kittens, unicorns and rainbows, and at the end of the rainbows was a magical pot containing not just solid gold coins but also an endless supply of chocolate-chip cookies and chewy fudge brownies.
But that's just me. Apparently nobody else finds the sudden appearance of this long sought-after option to be a big deal.
