Anyone find a secret command line 'default' setting to change how Mission Control / Spaces works so it shows the previews without having to go to the top of the screen?
I hate this change too, but I believe this was done to improve the frame rates. 10.11 finally has smooth Mission control animations on a retina display, even when a lot of apps are up and running, which is great.
I don't see any other reason why they would hide this from showing up by default.
If you use you display in some Scaled mode it could be the culprit of low framerate as it then runs at something like 3k by 2k resolution which is then downscaled to your selection.
I think the improvements are more around Metal and such. It see improvements everywhere not just Mission Control, especially when hooked up to my 4K monitor.