This is obviously a matter of opinion. Of course there are endless different email GUIs, on all the platforms; if you don't like one, there are many alternatives.
I cannot imagine that a "decent GUI" by modern web-development standards could be anything I'd prefer to what already exists. Email works and generally doesn't let designer ego get in the way.
How can you say that when it's one of the benefits of email that it's an open protocol and there's thousands of different apps built on top of it. Terminal UIs, web interfaces, chat interfaces etc. - that should be one of the cases where there's a GUI for anyone.
This results in the top and left-side bars staying decorative and unobtrusive, but the concurrent visitor counter thingy is floating well above the bottom-right of the page. But then, as I shrink in the page width, the left-side bar and the concurrent visitor counter swap sides, and also a bunch of the content from the top is added to a new bar at the bottom. Eventually, the horizontal space between the side components becomes irrelevant such that they function as an additional bottom bar. At which point, maybe 40% of the vertical space is used for navigation and other shiny toys. This is really annoying, and the overall experience is inconsistent across widths.
(Also, at all widths, the top-bar has a transparent background, so main page text interferes with top-bar text as it scrolls.)
All of this is done in CSS, so presumably it can be fixed that way too.