Geometry first, most importantly headtube angle, fork rake and wheelbase, which are the major determinants in how a bike handles. Although most road bikes are pretty much the same. Then component group.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1