Hmmm...
Frame and wheels. Frame for the fit and the style (townie, racer, endurance, touring...) but also the wheels as they have such a large impact on the overall feel; they connect all the rest and YOU to the road/trail. You can make a great change to how the bike feels with a wheel change. Just look at the pros- TT is one wheelset/type, climbing is another, another for the flats...
Tzvia- rollin' slow...
Specialized Ruby Expert/mens Bontrager Inform RXL
Specialized SWorks Safire/mens Bontrager Inform RL
Giant Anthem-W XT-XTR/mens Bontrager Inform RXL
Fuji Newest 3 commuter/mens Bontrager Inform RL
Novara E.T.A commuter/mens Bontrager Inform RL