In particular, reordering pictures within a gallery would be nice with the DOM.... just move the thumbnails around. (drag & drop being ideal, but Up/Down arrows would work, or clicking the top/bottom of pictures.)
I'll make a user pref for "use old HTML" always, and I'll also automatically go into that mode when UserAgent sniffing suggests fun DOM stuff will be impossible.
Target browsers are Mozilla (and thus Netscape/Galeon/KMeleon/etc) and MSIE 5+. Any DOM bug reports we get for other browsers go into our "old browser" useragent list, which will initially be populated with only Netscape 4, notably.
Summary: We're defaulting to doing cool new things, but we'll provide support for those old browsers because, well, it's the right thing to do.