Digga, I am going to disect your post point by point.
1: True, viruses have been written for both Mac's and Linux (as well as BSD and all other unixes) - the difference between a windows virus and a virus for a unix based operating system is the ammount of damage it can do. This is due to the very way Unix was designed. If you are stupid enough to use the root account, and visit suspect sites, run suspect executables etc, then you really do deserve whatever virus you get. Just as an example - out of the box, most users on a windows machine can install software, and that software is installed in a system directory (Program Files) - they can also delete and change files in said directory. Out of the box, the same cannot be said for unix. Users have the ability to run said files, without the ability to change or modify them.
Then there is the fact that 99% of people I know (which leaves the 1% of machines I set up for people I know) used an account with Administrator priv's. So any security that was built into the system (ie permissions) were laid useless by the fact that out of the box, "Power User" and "User" accounts were useless. The solution - which has been implemented on OS X and Unix in general for the last god knows how long is the ability to temporarily switch into "SuperUser" mode (aka sudo), so that when an application needed higher permissions, you were asked for a password, and it had to be a concious decision.
I wont deny that Windows has taken a few steps forward in the latest release, and I have not had time to adequatly test it, so I won't comment, but it's reputation as an insecure operating system is well deserved.
Point number 2: Incorrect. All software houses/people willing to fork out big bucks (for MSDN) have had access to various release candidates and betas for Vista, not EVERY DEVELOPER. Many software houses do not make the sort of funds to justify said subscription. Also, software houses are not required to release update patches for their older software because of a new operating system, unless it stated in the system requirements that it works with Vista (which some did prior to release). Software houses exist to make money - and there is no money to be made in spending X thousand dollars writing patches to freely distribute to customers who have upgraded to vista. Especially if their product was a 'once off' type purchase and they stand a good chance of people buying the "Vista Upgrade Patch" or "Software X - now supporting Vista".
Third point: Correct. In a way, this is how software evolves - through a process of leapfrogging the competition - they add feature X, we add X and Y, they add Y and Z, we add Z and A.
Fourth: Not quite correct. See the one thing that Apple do is provide a well tuned system. OSX is optomised to run on the hardware they have selected. You can get "tuned" windows systems too, and they cost about the same - if not more. That is why when a Mac G4 600MHz was put up against a 1.8GHz pentium 4, the Mac G4 seemed to be faster. I did a few design courses during highschool, and we had a number of G3's, G4's (running OS9 and OSX) and MS systems (XP) - The G3's and 4's always outperformed them much higher specced XP systems. That is what you pay for.