I don’t see a big visual improvement, but the overall simplification of the UI and reworking the previously atrocious underlying HTML code is a step forward.
Not sure why they went with an XHTML 1.0 Transitional DOCTYPE instead of, say, HTML5, which is practically tailor-made for a website like Wikipedia. I’m assuming that the new layout has been in the works for so long that HTML5 didn’t seem like a viable option at the outset.