tl:dr – general usability improvements, HTML editing for the DOM, timeline (a lá the beginnings of Speed Tracer), audit (a lá YSlow). Wiki here: http://trac.webkit.org/wiki/WebInspector
I have been playing with the Chrome/Safari Web Inspector for a few weeks now and let me say that I kind of like Firebug more.
I hope this update make CSS editing more Firebug-like.
I prefer WI for some tasks, Firebug for others, but with every update I feel I need FB less and less.