Ever since I was a kid I was fascinated by the idea of three way calling, talking to two different phone lines at once. I fondly remember in 1985 being hacked into some phreak conference call that also had chains of three way calls hanging off it, I felt so cool.
Thanks to the magic of iPhone three-way calling is easy enough that I can actually use it. Most cell phones have been able to do three-way for years but I've never figured out how to do it with a traditional awkward phone UI. I'm always afraid I'll hang up on someone rather than flash the hook or I can't figure out how to rejoin the calls or whatever. It's just too hard.
Three-way calling couldn't be easier on the iPhone. During a call the screen has a bunch of buttons on it. Tap the "add call" button to put party #1 on hold and call party #2. Now there's a button "merge calls". Tap it and now everyone's talking. The screen even shows you the contact names of both parties.
Amazing how easy things can be when you have visual feedback and more input mechanisms than almost hanging up on people.