Short, sweet and nicely captures the problem of quoted sections in email:
> Because it messes up the order in which people normally read text. >> Why is top-replying such a bad thing?
>>> Top-replying.
>>>> What is the most annoying thing in email?
(badly adapted from this post)
On the other hand, if you reply at the bottom people have to scroll all the way to the bottom to see where you brilliantly typed "ok". :-)
This is true for some communication styles, but not all. Imagine you open your inbox and read the following four messages in a row.
Message 1:
What is the most annoying thing in email?
Message 2:
> What is the most annoying thing in email?
Bottom posting
Message 3:
>> What is the most annoying thing in email?
> Bottom posting
Why is that?
Message 4:
>>> What is the most annoying thing in email?
>> Bottom posting
> Why is that?
Because it's annoying to have to keep reading the same crap over and over again if you just read it 5 seconds ago.
In business conversations, top-posting works really well, since you don't always have the time to do an adequate snipping. It lets you communicate quickly, but leaves a trail at the bottom in case anyone gets confused about the context.
Of course, responses to long, thoughtful messages should always be inline, so that it's clear exactly what you're responding to in the message.
Aren't blogs also displayed in that fashion? ;-)