Apple has received a lot of criticism for making the iPhone application universe a closed system. Every single application must be vetted by Apple's mysterious approval process before it makes it to the iTunes app store. This system is quite different from how Internet-enabled and PC-based applications function - where free-for-all rules rules govern how applications are created and distributed.
Although I agree that a closed system is highly flawed in many ways, it is also an extremely comforting mechanism. In a world where any web pop-up, email attachment, or even Twitter/Facebook link could lead to virus hell, it's very pleasant to not worry about whether an application will disrupt my iPhone.
No matter how controlling, limiting, and ultimately closed the iPhone ecosystem remains, it sure is very user-friendly - down to the inherent peace-of-mind concerning every single application download.