There are a couple of reasons, Vista will still have the same bugs/holes/problems as XP and every other Windows release in the past. Just having a "feature" doesn't equal usability. Apple may have a pretty face that everyone wants to copy, but underneath the pretty face is a well thought out UI. Viruses, spyware, security problems won't magically fade away with Vista either.
Well they are trying to make it more secure, they won't leave it running in administrator mode by default. That should help to prevent malware from working so easily. But windows users are actually used to this, I mean the typical attitude is ohh I just run a virus scan, what's the big deal.
But who knows what's going to be actually IN Vista. MS keeps throwing things out here and there so they can actually ship the damn thing...pretty soon Vista will just be XP with a new UI theme and that's it! lol
It's being released this year ready or not, microsoft says beta 2 will be feature complete which should be out shortly, rest of the year they'll just be finishing it, clearing up the UI and fixing bugs. Microsoft has decided that it's too late already and it must be out on the launch date whether it's a finished product or not. So that means they might do what apple did with cheetah. Release a very good beta.
I don't see Apple releasing Leopard at the same time as Vista. From a marketing stand point, its stupid. Vista is the latest MS release in years and news of it is going to flood the newswire, good and bad. Releasing Leopard at this time is not good because the news of both OSs would be diluted instead of Leopard getting its own spotlight (no pun).
I don't think that's true, this is not like releasing a movie. You're much better off not having your competitor get all of the spotlight, and some of the excitement about a new OS could rub off onto your product.
Vista = Windows XP with a new UI theme, a Spotlight-like search/index function (which you can get with Google Desktop), some GPU-rendered special effects, and Internet Explorer 7. The WinFX file system has been dumped, the legacy support has been boosted (meaning same old registry and .dll headaches), and other "interesting" features have been pared down.
They have a dashboard clone, sidebar with gadgets. And the GPU rendered especial effects, well you're forgetting that a major technology is behind this. Microsoft has basically cloned quartz 2d extreme and core image, they call it avalon.
There's a new explorer. New start menu, thumbnails for the taskbar and flip 3d. This is not a minor upgrade, this is the biggest thing to happen to windows since windows 95, with the majority of the UI being changed, all of tiger being copied

and a whole new graphics subsystem.
Big F-in' deal. Have you seen the UI for IE7? God is that awful! The menu bar is BELOW the tabs. WTF? Is the menu really going to change depending on what the browser tab is showing?? Knowing Microsoft, it probably will
The menu bar is in such an odd stop because windows explorer doesn't have any menu bar. And when IE is used only does it come up.
Microsoft is trying to move away from the menubar as a UI tool, you can see this is next microsoft office version as well which is going to use a flexible toolbar rather than a menubar.
Explorer, without a menubar:
http://www.winsupersite.com/images/showcase/5219_03.jpg
http://www.winsupersite.com/images/showcase/office12_b1_word_03.jpg
Also windows media player and msn messager are other examples. They think that a toolbar and context menu are a better way to input commands into a program, and it's possible the menu bar will be taken out of IE7 in future beta's.
Anyway, I think vista is going to be a huge improvement over XP, and even though OSX will probably have a better designed and more usable UI. It's really not going to have any better features, so leopard is going to have to do something great, because typical windows users are going to be blown away with vista, as it offers a lot more over XP.