I’ve often said that greater possibilities for addressing resources and points within resources means greater possibilities for linking, and the new predefined attribute xml:id can make a lot of straightforward, simple addressing of points within documents easier.

There are apparently some potential architectural problems, though. If this proposed W3C spec got this far under your own radar, you owe it to yourself to check out discussions of its good and bad points by David Megginson, Norm Walsh, their readers who contributed comments, and, more recently, Elliotte Rusty Harold.