From multiple customers we got the feedback that the difference between a group layer an the sublayers is not that clear. Of course there is already a difference with a small indent for sublayers and a arrow to open up a group layer.
However suggestion that are made to improve the visibility
1. Make grouplayer bold
2. Make grouplayer circle instead of a square (becomes maybe different with nested group layers)
3. Increase the ident a bit more, or make it configurable
In addition it would be nice to differentiate in symbology if all sublayers are active or a subselection of all the layers. I.e. a circle if a subselection is active and a square if all sublayers are activated. This gives a better overview to the end user.