When in the PM's Inbox (and likely the Outbox, too?...): When you mark something's LABEL it now takes you to that PM. It used to keep you right there at the Inbox/Outbox listing display.
Now I have to scroll back up to the top of the page to get back to the listing instead of the posts themselves.
(This is a real pain if you're marking several PMs, each of them twice.... (e.g. Remove a label, add a label; Next PM, remove a label, add a label, ....)