... why people felt obliged to yell into their mobiles, until someone explained to me that mobile phones, unlike regular landline phones, don't provide feedback into the earpiece. In a landline phone, what you speak into the microphone is played back to you through the earpiece, thus discouraging you from yelling into it (as you'd get an earful of your own words). But in a mobile phone, this doesn't happen, presumably for some fiendish technical reason.

I certainly don't look forward to mobiles on planes for that reason. Maybe they could charge very high rates for calls but very low rates for texts?