These upgrades benefit performance/function directly (in decreasing order):

- Installing better brake pads (Kool Stop or equivalent)
- Changing to hyperglide freewheels
- Putting good rubber underneath (I like Paselas or Continental Classics)
- Using lined brake and gear cable housing with die-drawn stainless inner cables

These upgrades are for comfort/fit/convenience:

- Changing stem to proper reach
- Changing saddle to whatever one prefers
- Changing pedals to whatever contact/clip/clipless system one prefers

On my 1982 Fuji Team, I changed out the rear derailleur from a perfectly serviceable SunTour ARx to a divine 1st Gen Cyclone, same with the front derailleur(s). The shifting is spectacularly nice - but this wasn't a needed upgrade. I did it for vanity's sake...

I also prefer SunTour power shifters on the downtube to the BlueLine or Cyclone shifters. That's due to improved gear retention under load, preventing unwanted upshifts.