Windows Vista's UAC was to ask "Are you sure?" for everything, not just running applications. There was no whitelist of secure apps short of what was installed with Windows.
The major difference with Mountain Lion is Apple actually has an App Store which 99% of the people who need this feature will use by default.
Everyone else will turn of(or turn down) this feature just like they did with UAC.
Changing one setting so I don't get emails from my mother saying she ran virus.app and now her computer is acting funny is worth it to me.
No, 99% of people will be using whatever they used before to get programs. Because these programs won't be in App Store, at least for quite a while, so people will get trained in ignoring those warnings and turning them off.
The direct consequence of the system that brands thousands of legit and safe apps "dangerous" would be desensitizing of users to this system and training them that the warnings system gives mean nothing and proper reaction to them is turning it off. You yourself would do this once asked to install new IM program that somehow isn't in Apple store. And then virus.app would have absolutely no problem running.
The app does not have to be in the Store, it only has to be signed by Apple. The default setting is to allow any app that has been signed by Apple to run. If a developer does not have the time in the next six months to sign their app then I do want a warning.
I agree with you in that too many modals can desensitize a person but let us look at what would need to happen to occur in this space.
Your average consumer would buy their new Mac with Mountain Lion and would immediately be prompted to look at the App Store. It is right in their dock and constantly promoted by Apple. This person downloads an app or two that they heard they MUST get (through friends/family/colleagues whatever). Some of the apps the person hears about are not in the App Store so they go to Google. Maybe they've been told about Adium or VLC or Chrome or Firefox or any of the other extremely popular Mac apps that have active developers. These apps are all signed by Apple because it takes like 15 min to get a certificate generated.
So now let's say we're at 4-8 apps downloaded. I would say a case could be made that your typically consumer isn't going to install 50 apps right after they get their computer but let us assume they download some app that is not signed by Apple. They are prompted with this warning and here is the valuable first impression and the learning experience. If this warning can properly convey to the user why the app doesn't start then you've won. The user will either decide they don't want to run "unsafe" apps or they will change the setting.
This warning isn't going to be constantly popping up because you wouldn't see this each time you went to run the app because you can't even run it. The only options are "don't run this" or "don't run this an get rid of it". The user can't just blindly click past this they need to understand it before they can advance.
Now let's say your typical use sees this and is scared and decides installing this apps isn't worth the risk. If for whatever reason they have been told to download a lot of unsigned apps they will then need to make a choice. Do I turn this feature off or am I OK with not having this app.
If you think the typical user is going to be installing dozens of apps from lazy developers then yes the user could be desensitized.
I believe that from past experiences we can see that even having to download a DMG is a jumping off point for so many people. Most users are going to experience this just a few times and if Apple controls the experience correctly then the user will be smarter and more protected than before.
Windows Vista's UAC was to ask "Are you sure?" for everything, not just running applications. There was no whitelist of secure apps short of what was installed with Windows.
The major difference with Mountain Lion is Apple actually has an App Store which 99% of the people who need this feature will use by default.
Everyone else will turn of(or turn down) this feature just like they did with UAC.
Changing one setting so I don't get emails from my mother saying she ran virus.app and now her computer is acting funny is worth it to me.