There are a couple really interesting things about the design of Gmail. One of them is that you seen entire threads of e-mail correspondence on a single page, rather than (as with conventional e-mail programs) clicking through one message at a time.
At the bottom of the page, there’s space to write a reply to the last message. This is one of those little design differences that– for me, anyway– has a substantial behavioral impact. I’m finding that I’m a lot more likely to immediately reply to a message that I get through Gmail than any other account, thanks to that little box.
Of course, hitting the “reply” button isn’t a high barrier to replying; but the box at the bottom of the screen is just a little easier, in a way that makes a behavioral difference.