The it factor. No one can truly explain it, but some bikes feel WAY better than others. I don't think it is just the fit of a bike, though that makes all the difference in the world on a road bike. But, certain bike brands continuously shock me with how great their bikes feel/work/function. Oh, by the way, we are talking about road bikes, definitely not mountain.
Through the years, the sleeper brands that come up are repeatedly the same: Miyata, Centurion, Some Fujis, Lugged Treks, Panasonic (hehe). After we repair these bikes, we test ride them and it is always the same response. Holy $*!^ these are SO nice, why did they ever stop making them? And moreover why do these feel so so so good to ride? I have tried to note some of the things that I have seen that makes them feel good. It still doesn't all add up to me, but it is a start of a list of things to look for when scooping up a garage sale or craigslist bike.
1. Typically, these are built from nothing but double butted steel (thicker where the lugs are and thinner in the middle with no loss of strength). This cannot be the it factor alone because plenty of "custom" frame builders use double butted steel and their bikes feel like so-so frames from anytime in history.
2. The geometry (of the road bikes) is typically similar in that the seat tube from the middle of the bottom bracket to the middle of the top tube is within 2cm of the length of the top tube center to center. What does this do for most people? It forces them to sit relatively upright; allows the weight of the hands to be nearly even with the weight on the rear-end. That is comfort! (note: this seems to be happening on Japanese lugged steel from mid-80s on, which the Italian manufacturers copied quickly)
Everything else doesn't matter from what I can tell. The parts hanging on the frame haven't ever made a bike for me. The frame is the glue pulling it all together. I have ridden some bikes that I am sure were not sold for very much brand new that felt better than almost any of the custom bikes I have ridden.
That said, the reason Salvagetti carries the brands we carry is just that: Black Sheep, Jamis and Giant all feel "it". They follow different rules then the old lugged manufacturers, but still get to the same place. We are very lucky to carry exactly what we love. But, the factors that led us to choose them were simple: The bikes have the it factor and people can afford them.