I have lots of issues with the current Wikipedia layout and the Winter prototype appears to fix/improve on nearly all of them! This is great work. Even though most of the time I spend on the site is in edit mode, I instantly appreciate the changes to the read mode.
Out of every single project I've seen, I think *this* should be the priority for the WikiMedia Foundation. It profoundly affects the way the site is seen and used - for the better. I think the dated look and basic functionality of the software is underestimated as a factor in known declining participation.
Only one suggestion - when entering the edit mode, the right rail area remains empty. It would be much more preferable to use this space to either (a) extend the edit text view to a width similar to the current one (in edit mode, the sole focus is the editing area and presentational spacing for improved prose comprehension is not a factor) , or (b) move the most useful editing links/tools/toolbars into this area.