I remember the first time I used an iMac: I spent about half an hour trying to figure out how to eject a CD. A context menu might not look very elegant but it's insanely useful and powerful and very easy to get the hang of. In the end I had to be told/taught that the way to eject the CD on a mac was to move the CD icon on the desktop to the trash (in the days of CD-RW drives, you'd think this would erase the CD).
That, to me, is the sheer definition of an un-discoverable feature.
The iPhone's lack of back button (and the fact that apps are very inconsistent in their back button placements) has brought me similar pains in getting simple things done.