- "disappear" beneath the rider
- feel fast
- not absorb the rider's energy
Fit. If the bike doesn't fit the rider well, the inconsistencies will constantly distract from the experience; even if at the edge of awareness, the (lack of) fit will prevent the rider from integrating fully with the bike.
Feel. Assuming the bike fits, it should have good "handling qualities." Desirable handling qualities may be different depending on the intended use of the bike -- a touring bike needs stability more than responsiveness; a criterium bike should respond instantaneously; a stage-race bike should be somewhere in between. How the bike responds when climbing, descending, cornering, accelerating and braking should all be in harmony. These characteristics are principally determined by frame and fork geometry, but also by handlebar width, stem length, rider size/weight, wheel and tire characteristics... and the list goes on. In addition to handling qualities, feel is determined by the ability of the bike to absorb road shock and vibration vertically, while not feeling like a wet noodle or allowing the front end to feel disconnected from the back. Modern frames have been able to make huge progress in this area through the use of big headset bearings (1.5" on the bottom now) and composites, which allow the stiffness of the frame to be distributed however the designer wants.
Energy transmission. Back in the day, high-quality steel could lend "spring" to a frame; energy put in by the rider might be absorbed in frame-flex, but could be returned when the frame rebounded. Shaping of tubes, selection of thickness, and such craftsmanship could all contribute to an efficient and spritely bike, while avoiding something so stiff and/or heavy that it was uncomfortable. With the advent of composite frames, the bottom end can be made extremely stiff and efficient, while maintaining compliance in the upper frame to give a comfortable ride. Either way, steel, titanium, aluminum, bamboo or carbon-fiber, the challenge is the same: deliver as much of the rider's energy to the wheels as possible, while keeping the bike comfortable (or tolerable, if a race machine) to ride on the road surface of choice.
I offer these observations because I've been riding what I think may be the perfect bike (or at least, by far the best I've ever ridden, and I've ridden a lot of bikes) which has made me think a lot about just what makes it so good.
That's not to say I don't think there's still room for improvement! Component manufacturers in particular have a lot of room to continue development of their products... but that's a thought that really deserves a separate post.