I think it's easy for the HN crowd to inadvertently forget what computers / phones / tablets / TVs look like to ordinary people.
I would encourage people to come up with actual use-cases and pro-con scenarios rather than 100% "emotion mind" opinions.
It took me about 15 minutes to get my Android Phone infected by a malware by downloading just apps from Google Play. If Google is failing in such basic task for "ordinary people", all MS accomplishes is to annoy regular developers, making their apps "suspicious" to the public.
That said, Smart Screen is a useful feature, I do want to know if the program I downloaded was not signed by the developer. What I would like from it however is that it should show up on every install with either "This program is signed by Malbolge Inc." or "This program does not have a signature" (in bold on bright red background with big exclamation marks border); "Do you wish to install?". Every installer gets this popup, nobody is spared. For the unsigned case, maybe require two clicks - a checkbox and a button. Because I always want to know who signed the installer, just in case I downloaded a compromised installer signed by a certificate different from what I expect. It also makes the unsigned installer popup less scary in comparison, though it must be kept visually distinct from the signed installer popup. For individual portable programs that are not installed, you will also have a "do not ask again" checkbox. Again, this will pop up for every program at least once.