The change, tcman, is in the way the pane navigation works. For example, pull up a Genre/Artist/Album type view scheme. Now, leaving Genre and Artist set to All scroll through the list and choose an Album that you know corresponds to a certain Artist and Genre. In 165 the other two panes (Genre/Artist) will change to list only the fields that match the Album you picked. In prior versions they did not, they just kept listing everything.
For example, say I have an Album from a Soundtrack in my list, which has (obviously) a bunch of artists on it, and is spread over two or three Genres (say Classical, Alternative Rock, and Drum and Bass). Now, if you choose that Album directly in the Album list (all the way at the right of the Panes) the other two panes will change to only list Classical, Alternative, and Drum and Bass (and the Artist one will only show artists that appear on that Album).
Its actually pretty cool for that purpose, and it just seems more intuitive to me. As marko mentioned, it is also useful for browsing Images (as you can "filter up" from matching keywords to see what Albums they all fall into). Hmmm... I might just start using the Panes a little if this sticks around!
There you go... Hopefully all de-knobbed and whatnot.