These are good and necessary changes, but bring Vista to the same playing field that Unix, Linux and more recently OS X "enjoy", where you have to concern yourself with a whole new class of privilege escalation problems. This is something Microsoft has never needed to address before and bug hunters were never concerned with finding. When almost everyone runs XP as Administrator, what good is a locally exploitable hole that lets a non-Administrator gain Administrator privileges?
Now, as this article at news.com.com.com.com shows, these holes are just now starting to be investigated, and are already being easily found. And comparatively few researchers and blackhats are looking hard at Vista, since it is so new that hardly anyone is using it now. Certainly many such bugs remain to be found, and given Microsoft's track record on security and the likelihood that their programmers have never really concerned themselves with this class of bug before, it seems quite possible that breaking into a Vista system will be almost as easy and common as breaking into a Windows 2000 or XP system. The only difference in technique will be that in addition to the initial bug that allows gaining local user privileges, a second bug will need to be used that then escalates that privilege to Administrator. After that, the typical rootkit or virus installation can take place as before.