The Skilltree Menu icon can be seen atop the skill tree in [http://www.diii.net/gallery/showphoto.php?photo=2649 various screenshots]). The Quest icon is a chalice, presumably meant to represent the perpetually quested after mythical Holy Grail. It's not entirely clear what the party menu does, since there is no friendly/neutral/hostile setting in Diablo III, with the removal of non-consensual PvP. Perhaps it exists to show where other players are, or to allow whispering or squelching.

The only functional change from Diablo II appears to be the removal of an icon for the message log in Diablo III. Also note the earlier version of these icons in Diablo III: they were in color and above the belt interface (same location as they had in Diablo II) as of June 2008. (Screenshot below.)

====Updated Icons: August 2009====

Here's the updated display as of August, 2009. Unfortunately, Blizzard has not released a full quality view of this, so we must make due with a blurry photo someone took of the game on a monitor. The image you see here has been enlarged and the color has been slightly corrected.