Instead i find the UI clunky and jarring. Look at the arrows indicating either sequential or parallel projects, those hideous little archive box icons, or the constant resizing of the main window when switching between perspectives.
Chris, can you go into more detail about what it is you dislike about the sequential/parallel arrows or the SAL shoebox icon?
As for the window resizing, that can be controlled with perspectives. Simply set the size you want and resave your perspectives. Alternatively, I think you can just uncheck the Restore Layout option in the perspectives window.
With the right font choices, OmniFocus looks a lot like many other apps in OS X, like Mail, Finder, Yojimbo, or even iTunes, at least at a superficial level. Doesn't it?