Is Acrobat "extensible"? In other words, is it possible for non-Adobe people to write add-ons/extensions/macros (whatever you might call them) that modify the behavior? If so, perhaps someone could write one for this.
I don't think it's just about not knowing how to open the Comments pane (there are three ways I know of - the arrow to expand the Tools pane is only one), but about convenience. I'm on the receiving end of many documents that have comments I need to step through and read, and I always need to see the Comments pane. Initial View would not help, because the documents didn't have comments when they were created - the comments were added by others later (in my case, magazine proofreaders, design reviewers, etc.). So here is an alternative idea to accomplish it: Include an option in Preferences that would show the Comments pane automatically when a document with comments is opened. Or both could be implemented.
I think Kelly Vaughn is talking about two different features: (1) the one described here (and at https://acrobat.uservoice.com/forums/590923-acrobat-for-windows-and-mac/suggestions/33091900-make-comments-toolbar-open-automatically-by-defaul and https://acrobat.uservoice.com/forums/590923-acrobat-for-windows-and-mac/suggestions/19479121-setting-to-open-sidebar-with-comments-by-default), which is set by the user who views the PDF, and (2) Initial View, which is set by the person who creates the PDF. Both are nice ideas, but I think the former is more helpful, as the setting is under the control of the person doing the viewing, and it only needs to be set once. Initial View would have to be set by the person creating the comments, and would have to be done for every document.
I too really want that. Of course it shouldn't open if the document doesn't have comments. And it should probably be a checkbox in Preferences so that each user can decide if they want that behavior.