Back in the 80s when screens were tiny and multitasking wasn't very developed, it made some sense to have the single menu bar. But there is a good reason why Amiga OS 4 is the only modern OS I can think of besides OSX that still has the single menu bar idea.
When you are full screen working on one program, it doesn't matter at all, but if you use multiple things at once, and have something off in a small window and have to go right up to the top of the screen for its menu bar, it doesn't make sense. The UI isn't being helpful, the closer the controls are to where you need them the better.
As such, there is nothing I'd take out of Mac OS as being something in a dream OS, as the rest of it isn't exactly special.
I also don't get random transparency, if I wanted to see my wallpaper all the time, it'd be on my wall as actual wallpaper. Nor do I like this massive icons trend. I also don't like the dock, it is too big for what it achieves, the Start bar just seems to make a lot more sense, especially when combined with a quick launch toolbar.
That said, I'm quite a fan of GNOME's solution, two menu bars, one at the top to be a spread out start menu (Applications is like the All Programs menu, Places is things like My Computer/Documents, and Settings is for the Control Panel and logging off/shutting down) and the bottom one as a taskbar. It uses less space compared to a dock + menu bar, but is less cluttered than the windows equivalent.
http://library.gnome.org/misc/release-notes/2.24/figures/gnome.png
For my current usage, it just needs to be Windows underneath, as much as I like the interface of GNOME, I can't run Linux/BSD all the time (though looking around I've found that VirtualBox may fix some of my current problems). So that leaves me just wanting the GNOME shell on Windows, which is actually possible in theory.
I don't dream to have a fancy desktop, I just want something where I can be most productive, which is the whole point of a computer.
So to answer the question: the interface of GNOME, the speed of BeOS, the stability of concrete and the compatibility of Windows.