I've looking at https://www.apple.com/osx/elcapitan-preview/, and I can't see one thing that looks interesting to me. Is there anything exciting, for a developer type, that they are excluding?
More seriously, from that list at least for me: - Speed and battery life from Metal, to start - Better window management - Bug fixes - Spotlight improvements (if you don't use Alfred already) - Audio indicator in Safari Tabs
Although I will likely wait and see whether all of my dev tools are going to shatter by letting someone else at work do it first :)
For example, it's almost impossible to see if a control is disabled or not in Yosemite. Or the default, white, borderless buttons on don't work very well on a bright background.
In El Capitan they fixed a lot of these problems. All buttons now have a border, and their state is clearly visible.
Split view support sounds good. Let's see how it measures up against things like Spectacle and Amethyst.