One of the wikipedia's largest design shortcomings is its inconsistency in look and feel before and after a user logs in. (note: to see a page in the "logged in" state, one must first sign up for a free account at wikipedia. The functional difference of being "logged in" is that the user has three new sections of links available on the side-bar: "context", "my pages" & "special pages".
However, the look and feel of the page completely changes when one logs in.

The list of languages (written in their own characters) is moved from the bottom of the side-bar to prominent positions at the top and bottom of the page in the "logged in" version. I can see the point of putting these lists of languages on every page, as I can imagine being rescued from a page written in Mandarin with an "English" link. However, it seems logical to tuck this list away at the bottom or bottom left of the page, especially once a user has logged in (and presumably did not arrive there by mistake).

The "logged in" version of the page also completely does away with the wikipedia logo, and replaces it with a hastily thrown together top-bar, whose blue awkwardly matches that of the links on the page. The "WIKIPEDIA" heading is written in a plain font rather than the distinctive one that accompanies the logo on the not-logged-in page, and I am hardpressed to find any similarity near the top of the page aside from the name wikipedia itself.

One thing the "logged in" version got right is the placement of the "search bar", which is logically placed near the top left. Despite being billed as the prefered way to edit, gone are the tabs for "article", "discussion", "edit this page", and "discussion", which are instead scattered among the poorly organized new categories on the side-bar. I can see why a few of the new links on the side-bar are necessary for those who are logged in, but they could have easily been incorporated into the already decent design of the not-logged-in page. It seems the designers thought it would be confusing to have more than one place to click "edit", arguably one of the most important features of wikipedia, yet decided to take up much of the screen with foreign language links. Thus, the design fails miserably in its "call to action".

Perhaps the reason that the designers chose such a different look for the "logged in" page was to visually set it apart from the regular page. This does seem like a good idea, as otherwise one might become confused as to why the links are different or whether the posts one makes are anonymous or not. Instead of completely changing the design, I would suggest a more subtle change such as changing the background color from grey to light red or green, thus hinting to the user that the mode is different while still keeping the layout consistent.