Note: The following behavior is what I’ve seen in Acrobat 7 and 8 on Mac OS X. I’m not sure if it works better in the Windows versions of Acrobat. Also, this technique may not apply in Acrobat X and later because the comment list interface is slightly different.
You can select multiple comments in the Comments panel in Acrobat, but not in the way you’d expect. Let’s say you want to set the status of three comments to “Completed.” Your natural inclination would be to Shift-click or Command-click them, but it somehow doesn’t work like it does in other programs: even though you’re holding down a modifier key, the only comment selected is the last one you clicked.
To select multiple comments, you need to click just below the top edge of each comment while holding down a modifier key. For example, to select a range of comments, click the first comment you want to select, and then Shift-click just below the top edge of the last comment you want to select. It might take a little practice, but you’ll get it.