I previously filed a bug report about the behaviour of comments when you click/double-click on them and was told that the developers had reviewed it and were not going to change it.
Honestly, Acrobat is the worst piece of commercial software I have to use, by some margin.

I Just compared with Acrobat X: There you have 20 comments in the window at glance and in the former version it's just 9 comments! And if I click on a comment it's gets a bit bigger what leads to slipping of all comments, when I click on the next. So it's hard to orientate. And I have to scroll much more than in X!

I was having trouble distinguishing one comment from the next, and not really thinking about it much, thought it was just me!. But yes, since you mention it, separation between comments and posters seems to be reduced or eliminated. I also remember different colors for different commentators. The interface is harder to use now and has slowed me down.

Please explain how hiding a function in a menu and doubling the number of mouse-clicks required to add a checkbox to a comment has "made it very clean and intuitive to use".

Seriously, do the Acrobat team even think about this stuff before saying it?

Hi ,
In the latest update of Acrobat DC, we have tried to modernize the commenting experience and at the same time, made it very clean and intuitive to use.
As part of that, the checkbox is moved to the right click menu of the comment.

Open comment>Right Click>Add Checkmark

Please also note that both the sort and filter options are available for the check mark.
We are sorry for the inconvenience caused and will consider adding the check mark by default for future updates of Acrobat DC.

However there is a workaround to add a checkmark in one step by using the shortcut SHIFT + K . Hope that helps for now.