For me personally, I find it visually more polished. On iOS, the UI elements have much finer detail, appear to almost have depth and far crisper graphics. For example, UI buttons have texture, almost seem raised where I find Android generally to be much more flat and simple. Best analogy I can come up with would be computer generated graphics vs. finely hand drawn pictures where iOS is the CGI. Also, for the most part, everything seems to fit together nicely from the OS UI to the individual apps while Android can have moments of disjointedness and inconsistency.
And please don't take these as absolutes, just rather my general perception. I don't find the differences between the two to be vast but certainly noticeable to my eye/taste and cleary, well, different. And to be honest, if I didn't find the differences so noticeable, it would probably be a lot easier for me to kick the iPhone to the curb entirely, considering it's many shortcomings. This is just one area where I feel it excels and personally carries quite a bit of weight with me.
Also, before I'm accused of being an iOS 'homer' and biased, I have used both OS extensively over the past year and am presently waiting for my new phone, an HTC One, to be delivered this Friday--can't wait!