I'm a Cannondale fan, especially of the mid-90s touring frames. Below is my take on the alleged harshness.
Only the third item in the list hasn't been mentioned before, and I'd like to know what y'all think about this.
(i) There is no vertical give in the rear triangle of any frame, aluminum or steel, when compared to the elasticity of the tires. Compressed air vs. solid metal, which one yields first?
(ii) The perception that "every bump is transmitted" is certainly there, depending on the details of the setup, but is mostly a function of tire pressure and frame geometry.
(iii) The aluminum frames sound different than steel, or maybe they "transmit sound" differently than steel does.
Any kind of rattling from the pannier against the rack is mighty loud on my T400, compared to my steel tourer, and it's not a pleasant sound.
Nobody makes bells out of aluminum, bike or church. I wonder if that is what causes people to perceive aluminum as harsh.
cheers -mathias