Personally the only thing I have against the current thing is the non-consistency. If you are to put the Start Game menu, the Brick menu and the Options menu together, you wouldn't physically tell they're from the same game. Except Brick/Options because same GUI textures, but that's it.
Also I personally don't like that Server Browser. When I browse a server I'd rather have a gigantic list of them instead of having isometric renders that make the scrollbars 12x times smaller, if I wanted to see the builds with a server browser, I'd use browse.php.
I would rather retain the Gui, but have something more modern. The current rounded GUI elements don't make up for it. I personally made myself some square GUI that I think looks more fitting.

All that it would be needed is new GUI textures,
alignment fixes for buttons and elements, and a rehearsal of the Start Menu.