I am always reluctant to download pre-release version when OS X get updated. So I waited for the final version of El Capitan in App Store.

• Apple Mail finally became a useable application in fullscreen mode: You can now start writing a message and go back to your inbox without having to close the editor window.
• It is now possible to have two fullscreen applications side by side (‘split view’ mode), and Spaces has been improved in many ways. Since I use Moom to manage my windows and desktop, I don’t see much difference, but I guess those little pixels lost in normal mode may be of importance for some people.
• It is possible to hide the menu bar in addition to the Dock! In the last release of OS X, Apple developers offer the possibility to change the color of the menu bar from grey to dark, and I find it a very good idea. I use a dark grey background and I never like that the menu bar pops out on the top of my screen. Now we can get a clean desktop free of any distraction.
• Apple introduced severe restrictions on its system files, headed under the term SIP. In particular, it is no longer possible to write to /usr/bin while many applications required access to this system path (e.g., TeXLive or Stata). See also: Sorry, Unix fans: OS X El Capitan kills root.

For a more detailed overview of new features in El Capitan, see: All you need to know about OS X El Capitan, or OS X El Capitan review: A swifter, more streamlined take on OS X.