Right now one can move up and down with the j/k keys, select an item with the x key, and then tag with the l key. I would like to skip the middle step and directly tag the currently highlighted item with the l key (without having to select it first with the x key).
That’s Gmail behaviour and we actually implemented it that way following requests from our early beta users. I guess we can think about skipping this step for the special case that you just want to label the item that is currently highlighted but not selected.
One problem would be the behaviour is inconsistent. If you are used to be able to label the currently highlighted (i.e. blue bar on the left) item with “L” you would be surprised it does not label the highighted item when you have selected (i.e. checkmark) one or more other items.
Gmail is a bit different in that you often are browsing in the individual email view. That is, typing L will label the email you are currently reading. Since you don’t really “open” Paperpile items in the same way I see how there might be some confusion, but I’m actually used to working this way in Gmail, since I usually file emails when I’m reading them, not from the list view. One option might be to have an option to move through items one-by one (I know I’ve suggested something similar elsewhere, in reference to a preview option), so as to avoid the confusion that you mention.