OSX El Capitan - Spaces bar - Show expanded window thumbnails/previews by default?
Solution 1:
Please go to http://www.apple.com/feedback/macosx.html And submit feedback, telling them you want (like many of us) this functionnality back.
Solution 2:
An available workaround is using the latest version of BetterTouchTool (BTT).
You can assign the new "Mission control & Immediately show desktop previews" action to a gesture (3-finger swipe up).
This does what expected, opens Mission Control and then expands the Spaces Bar. However the second animation happens only ofter the first one has finished, so the windows move about a bit.