I agree mostly on what you say about OS diversity, if, let's say hypotetically 30% of people use Windows, 30% some Linux flavor, 30% Mac, 10% other OS, it will be quite harder to determine which system to target. Today is faily simple, if I want a bot-net I will focus on Windows and IE, because I have about a 80%~90% of the systems at my disposal.
Also, Windows desperately needs to restrict User access and make BIG ANNOYING WARNINGS when you are connected as Admin.
Most people I know that uses windows are always logged on as admins, because it seems they install/remove drivers, applications and so on all the time
I think this is the biggest problem of Windows now a day (and that somehow it self-destructs given enough time)