2 should really be 1 since "0" was 1, but version numbers on web techs are always a mess.
the dom as of 4 years ago is dom2: http://www.w3.org/TR/DOM-Level-2-Core/expanded-toc.html
most print media refers to this dom, and most of the features are still used today.
dom2 basically solidified the existing Netscape and MS doms. I'd say adding namespace support was the biggest over-all change. i can see it trying to use the same interface for html and xml, and being released ion 2000 hot on the heels of xhtml, you can guess that xmlishness was indeed the main focus. i think normalize(), textContent, replaceChild(), and isSupported() were first codified in 2, if memory serves me correct.
I still people using Node.data to this day; just because something's formalized, doesn't mean it's popular.
dom3 added children, draggable, dataset, classList, querySelectorAll, and a few other nice methods.
we are now on 4 if anyone is still counting.
the recent changes basically tone down Node(), so that attrib nodes and the like are not still referred to by iterating just dumb nodes. also form validation, nextElementSibling, and a few other lazy methods were added.