Which components actually make a difference?

This little gem of a review of a BLB hub
and its "it has almost the same performance as a real track hub" got me thinking: how can you tell? It's a hub. You lace it to a rim, put a sprocket and lockring on it and it goes round. So the question is: which components actually make a substantial, immediate difference to the way a bike performs and which don't? Not components that might save a few grams or make you a couple of seconds faster up Mt. Ventoux or produce an extra couple of watts on your PowerTap. Which components, when improved, actually make a bike feel better then moment you start riding it? Comparisons only of similar condition items please (no comparing rusted old junk with brand new ) and within reasonable limits (no comparing plastic BMX wheels with Zipps). Note that collectability, aesthetics and durability are not factors here, those are all good reasons to spend more; immediate feel and speed are what counts.

List (and argument) time:

Do make a difference:

• Saddle - obvious source of discomfort if you get it wrong
  
• Grips/Bar tape - see saddle
  
  
  
  

Don't make a difference:

• Cranks - they're effectively a metal bar, unless you're a monster even a cheap crank will still get all your power to the chainring)
  
• Chainring and sprocket (for a fixie) - if the chain stays on them the quality makes no difference.