when vista came out, clearly one of the biggest pain points to end users was user account control (UAC). this is the feature that requires authorization to run certain applications which would require elevated privileges...most notable being installations. osx, of course, has had this feature for a while. osx requires the user to authorize elevated changes like changing account information or other system settings, as well as installations. osx differs slightly in that once you authorize in certain areas (like sys prefs) it remains 'unlocked' until you lock it again explicitly or other actions do.
another thing that vista (and actually xp sp2) introduced was protecting users from downloaded files. if you downloaded a file and then tried to use/execute it, vista would prompt you that you downloaded the file and should be cautious, etc. a good feature IMO. after prepping my clean pave of leopard i downloaded a few of my apps and when launching one saw this:
looks like leopard implemented a similar feature. it is good to see security taking a priority in updates to operating systems. sometimes they are annoyances (i actually think this one is in vista and in leopard), but i get it, to the 'lay user' software developers have an obligation to do their best to protect the system, user and networks while at the same time balancing that with proper user experience. i've seen other leopard security improvements as well that catch up to some of the introduced features in vista...good to see it taking a front seat, or at least i think so.