My modern bikes always need a little ol school flavor. I usually use older cranks like magics, cook, or Carambas just to spice them up and still am a sucker for anno. I always need the skewers, chainring bolts, headset and seatpost binder to have a matching color.
it's been said already, but i come at it from the other direction...it's not about retrofying a modern bike, it's about modernizing a retrobike where there are significant improvements to be made.
eg: a modern fork and front disc brake. modern tires. and, well, that's about all i can think of for must haves! maybe triggers instead of thumbies, but i'm an SSer, so i don't have that debate.