It's hard to get because iphone users want it to be an open general-purpose computer that they own, and it masquerades as one most of the time. Remember when the apple-using-world lost it's collective shit over a free U2 album? It's not like they didn't know _intellectually_ that Apple had the ability to do that - they just hated being reminded of it.
All of them. A general purpose computer (e.g. a linux box) is one that can run arbitrary code. A special-purpose computer (e.g. an ATM machine) only runs what the manufacturer intends it to run. The former is powerful-but-dangerous, and the latter is safe-but-limited.
Customers, of course, don't want a trade-off - they want power and safety. IOS is an attempt to get as close to that as possible, but it only works until iphone users want to do something on their phone that Apple doesn't want to let them do - that's when the illusion comes crashing down. When this happens, Apple users don't say, "Well, I'd like to run XYZ on my phone but I understand that I can't because that's the trade-off I made in exchange for a nice app store with no viruses in it," they say, "What the hell! I should be able to run XYZ if I want, it's my phone!"
What does general purpose mean? It means I get to do what I wan.
Anyone who complains that they can't do something that Apple won't let them is a candidate. Anybody who jailbreaks is definitely one. There are plenty.
