Here's how you can reply to iMessages from within notification alerts and banners in OS X Mavericks. This is a very fast way to send a reply, it's much better than having to click on the Messages application on the Dock.
1. Go to System Preferences.
2. Go to Notifications.
3. Click on Messages.
4. Click the Show in Notification Center option.
5. Choose the Banners or Alerts option.
An alert displays the Reply and Close buttons, but the banner will require you to put your cursor over it to see a Reply button.
Read more: Tech Radar