OK, so so far for me, the Verholen barbacks I added to my K13 have made the biggest difference for me.
They completely changed my ability to keep weight off of my arms and thereby allow me to better control the bike. Too much weight on the arms = stiff steering; less weight = much improved handling agility.
Behind that, I'd say the extra brake lights (lite buddies), the rear signal conversion kit (LEDs vs. bulbs, with running, tail and stop lights), my front driving lites, the rear bag reflector kit and my helmet halo were the next more important add ons--I'm pretty fanatical about visibility. Multiple points of light and reflectivity are critical on a bike, IMO.
Last (but not least), even though they aren't necessarily farkles, I'd say the heated grips and my kanetsu electric vest. I you aren't uncomfortable, you can pay much more attention to what's going on around you.
You guys have a valid point, a modern motorcycle is much safer than an older one...but, the intent here is to help everyone learn what changes you have made to your current ride that you see as vital.