Playgrounds introduced some significant changes with Beta 6. Most importantly, the timeline slider appears to have disappeared.
Update: Norio Nomura found it:
@ericasadun “Show Timeline” option exists on File Inspector > Playground Settings.
— Norio Nomura (@norio_nomura) August 26, 2015
You can click on intermediate calculation points in an embedded value history to view those values as tiny pop-ups.
The Quick Look system is redesigned for both the value history and the results gutter. Instead of the old-style [=,~,o] buttons in the embedded value history, you now either see just a single value, a graph or a list:
Update: Kelly G writes in the comments, “Also if you Control-Click on the inline results of a loop, you can toggle between Latest Value, Value History, Graph.”
The only hover-to-reveal action that remains is for list Quick Looks:
You can no longer close value summaries in the main timeline. Click the filled circles in the gutter instead.
The Beta 6 version of the Playgrounds Book is updated and available now in iTunes. I’m pushing another update with a few more errata today that should hit the store tomorrow or the next day, and I’ll continue updating as I find new features.