Multiple page moderation is good, but it might become a problem if you can't remember what's already been selected, or want to double check you selected the right items.
I think it would be better to be able to ALSO see an optional list of the selected threads somehow in the overlay. For example, click "show selected" and it pops up a list of the titles linked to the original threads, and a way to deselect them individually. And maybe a way to jump to the page it appeared on, in case you meant to select the one above/below it. This way if you are on another page and have second thoughts, there is an equally simple way to change/remove an item in the total selection without trying to figure out what pages "X threads" were originally selected on and without having to Deselect All.