I run into similar situations whenever I restart my browser. All of the YouTube tabs I had open from before start playing simultaneously (at any given time I often have some tabs with videos paused, ready for me to watch later). It's quite distracting and takes some effort to find and stop them all.
I think a good solution for both of our problems would be to not load any of the Flash content in a particular tab if the tab hasn't received focus since the application was restarted. Once I open that tab, the plugin can load at that point and then the video can start playing.
Even this becomes annoying when you have to wait for the page to load to see it's content and is heavy but not media content heavy. Something like Google+ or coursera pages.
Safari implements your solution. Though I wonder if it would even help if their Netflix tab was the focused one; for that case, it would also have to wait for some user interaction or something, which seems messier.
My solution to this is just to go into the Task Manager and kill the Flash plugin about 30 seconds after every restart (This saves a load of memory too). When I actually want to watch one of the videos, just reloading the page is enough to kick Flash back into gear.
I think a good solution for both of our problems would be to not load any of the Flash content in a particular tab if the tab hasn't received focus since the application was restarted. Once I open that tab, the plugin can load at that point and then the video can start playing.