I'm an avid iPhone user myself and I really love and appreciate the overall experience and the device itself.
But, being so successful, I feel that iPhone surprisingly has numerous undiscoverable UI features. Many of these, unless accidentally discovered, will never be known to a user. Also, many of them are not even listed under iPhone Tips & Tricks also! (I bet more than 90% of iPhone users wouldn't care to read the iPhone User Guides).
"iPhone Mobile Human Interface Guidelines" states that "iPhone OS... delivers consistent behaviors that users expect". This may be very much true. The UI could also have excelled in many usability studies too.
But for a layman user, to discover the UI features available and to use them is really difficult.
Here's a list of such features:
Double click lock-button (on the top) to reject a call
While there is an incoming call, click the lock-button to silence ringer
For 'Undo'ing something, shake the phone
Swipe to erase mails or list items
Press-&-hold icon to delete/move an app
To scroll to the top while reading, tap the annunciator
Touch and hold an image from the web to save it
Touch and hold on-screen 'Mute' button to place a call on hold
To restart phone, press-&-hold the lock-button and the home button together
You can 'clear' all; but can't erase a single Recent-Calls entry
I feel, the iPhone UI could somehow be bettered. Hope sooner or later it will...