The realization that Windows was stealthily making dozens of internet connections. I don't know what they were for and there was no reasonable way to disable it.

I'm an evangelist so far as to recommend people to switch to Linux or BSD. But people always find some reason to stay on Windows - "I will switch when Linux starts doing ". (Not gaming). It often feels like they're searching for a problem to justify not switching. I don't bother fighting with such people.

I'm obsessed with modifying Linux to my liking. Programming was another one of my reasons for switching. These days, I program components of my desktop to suit my needs.