Herein lies a problem with communication. Someone takes the time to write you. Social conventions obligate you to respond (though you'd be better off if you didn't respond to most). Even if you choose to "put it off for later," it weighs in the back of your mind.
Now fast forward to modern day, and the cost/time required to send an email is trivial. More emails get sent. Busy people get more inbound. Social conventions obligate you to respond, especially if it's someone you know.
We'd all be better off if the expectations of response were lower. That's why I like Twitter. There is little expectation or obligation to respond.
I can remember Terry Pratchett (notable British author) announcing having to abandon having a public e-mail address because of this. He'd had one for basically as long as internet access has been available to the general public, which worked well enough in the early days of the internet but...
I think the expectation of a response to an email is lower than to a letter. I certainly behave this way--if you send me an email, I might not respond but if you send me a letter I definitely will.
Of course, being a largely useless college student, I hardly get inundated with emails and so will probably respond in either case, but I would certainly feel better about ignoring an email than ignoring a letter.
I am willing to believe that the probability of an individual person expecting a response to an e-mail is smaller than for a letter (although I am not certain I agree). However, the cost of sending an e-mail is lower, so more people do it.
In my experience, the math then works out that numerous people are still angry if you don't respond to their e-mail; I have had people make entire blog posts about how horrible and dishonorable I am for not responding to e-mails that didn't even have any concrete questions or action items.
On the far end of this is Twitter: I get tons of messages on Twitter; enough that, when something is "happening" in the iPhone ecosystem, I have gotten hundreds of @replies in a few hours (in slow times, I thankfully get very few). However, Twitter only lets you get 800 messages from any non-timeline search, even using their website.
This means that if I'm not sitting around checking Twitter all day, there is no way I can even see all of the messages being sent at me, much less respond to them. However, in all of that mess, there are at least a few people who get really angry that I didn't see their message and respond to it personally.
Once, I spent a couple weeks, spending nearly all of my time answering e-mail. I still had people get really angry at me for not responding to their messages, and I now also had people angry at me that I wasn't accomplishing anything (such as writing new code).
Additionally, I was now getting much more e-mail, as the people I was responding to were taking that as an indication that they should send me more e-mail. That experience made me question the value of responding to anything at all. :(
Now fast forward to modern day, and the cost/time required to send an email is trivial. More emails get sent. Busy people get more inbound. Social conventions obligate you to respond, especially if it's someone you know.
We'd all be better off if the expectations of response were lower. That's why I like Twitter. There is little expectation or obligation to respond.