A lot has been written about Quick Reply, iOS 8's new feature that borrows extensively from the jailbreak community while retaining some minimalism of its own. With Quick Reply, you can reply to Messages, Tweets or take action on emails and reminders etc. right from the lock-screen or the notification banner. You don't have to open an app to reply to some notification from the app.
Quick Reply is extensive. Apple has integrated the feature with a lot of stock apps (along with integrated features like Facebook and Twitter). Here's an overview of how Quick Reply works on the lockscreen in iOS 8 and how you can make the best use of it.
First of all, only stock apps and integrated features (like Facebook and Twitter) have this feature. This feature is not available in (all) third party apps. At least, not yet.
Quick Reply in Message
When you get message notifications on the lockscreen, you do not need to unlock to reply. You can just swipe “left” on the lockscreen notification and tap on Reply to quickly reply to the message.
However, note that there are a couple of toggles that need to be enabled before the Reply button shows up. Read this post to find out what.
In case you get notifications from the Reminder app, you can act on it without having to open the app. Swipe left and you will get options to snooze or mark as completed. Snooze works just like the alarm clock's snooze while the other option will strike off the reminder.
iOS 8 added some more configurable options to the email app. You can now change the options you get when you swipe left/right on an email thread. While this is in-app, what is interesting is that it reflects on the lock-screen notification too.
If you get an email notification on the lockscreen, slide left for get options like Mark as Read and Archive. It looks like you can change this when you change the settings as shown in the linked post above.
You can apply same action while getting a Facebook or Twitter notification, though the options are different in the two. You get Mute and Reply as options on Facebook notification while on Twitter you get Reply and Favorite.
The Quick Reply is largely message/communication oriented feature and it is very useful when you are busy and do not want to unlock the phone. Makes for a very easy, intuitive and quick usability feature.