Was there really that big of a different between “via @user” and “RT” and the retweet button?

User interaction drops dramatically with each extra click required -- I don't think it's hard to extrapolate that an action that involves at least two additional clicks (select text than focus the input), plus keying in text, pasting text, and reformatting pasted text to fit fewer available characters would greatly drop the rate of retweets.

I think similar mechanism explains why some diseases blow up into epidemic and some fizzle out. Epidemiologists have all the hard math on just how much friction is needed.

Psychologically speaking, I always felt the original RT kind of forced the RTer to put the words into their own mouth.

Taking something that required thought and typing and turning it into a one-click, no-thought-required operation? Absolutely.

Jeff Bezos certainly thinks so -- why else would he have bothered patenting one-click buying?