What functionality of iOS can be accessed from the lock screen?
I'm including the ones mentioned in your answer in the name of having a complete listing in one place.


Toggle music controls (Play/Pause, Previous, Next, Volume) by double tapping the home button.
Take pictures and videos by sliding the camera button. 
Invoke accessibility features by triple clicking the home button. Configurable in Settings-> General-> Accessibility-> Triple Click Home. You can configure toggling VoiceOver, toggling White on Black, toggling Zoom, or asking which to do.
Make phone calls by tapping the Emergency Call button and dialing a number. iPhones only.
View snippets of emails, text messages, and other notifications by looking at their  notification preview. Configurable in Settings-> Notifications-> App.
Activate Siri to make calls, send text messages, send emails, get directions, check calendars, etc. by holding down the home button. Configurable in Settings-> General-> Passcode Lock. iPhone 4S only.
Take a screenshot by pressing the home button and sleep/wake button at the same time.
Power the device off by holding down the sleep/wake button.