The notifications in Android L have been totally revamped. In the revamping it seems that Google has forgotten to include a clear all notifications button. Maybe they have decided it is not necessary, or maybe it was an oversight. Either way if you are used to being able to clear all your notifications at once it can be pesky that the feature is gone in Android L. Koush found it frustrating enough to create and release his very own "Clear Notifications" app. This will add the clear all button back to the notification panel and relieve you of all of your frustrations! Head to the link below for the download.
Clear Notifications