Some of the coolest features in Gmail are hidden in the Labs section. Here, things that may need some polish or which would only be useful to a small number of people, are all put together.
Once in a while though, some of these experimental features prove useful and popular enough to turn them into full-blown ones.
The latest such Labs experiments to graduate to the full Gmail product are "Send and archive," "Quote selected text" and "Default reply."
"Send and archive," as the name suggests, allows to automatically archive an email as soon as they send it. You can enable the button in the Settings > General tab.
Likewise, there's now a "Default reply" which enables users to set it to "reply," as it is now, or "reply app." This option is also in the General tab.
Finally, the "Quote selected text" feature enables users to select the portion of an email they want to use and then click reply to have it included as a quote in the new message.