It's speed, it's simplicity, it's security, it's integration with supporting products both hardware and software wise, coupled with the sheer mountain of accessories available, it's App Store, it's build quality and looks, it's screen, it's gaming capabilities, it's battery life (which is actually pretty good for a smartphone) it's internet browsing experience, it's HD video recording/camera quality...
Can't think of anymore off the top of my head. I agree the Desire does most of those things to an equivalent standard, some it does to a sub-standard level and there are quite a few things the Desire does to a higher standard; hence why a Desire is my everyday phone and not an iPhone.
However, I still like it. If Android didn't exist it would be an excellent choice for me, despite it's short comings. I know it's easy to rant and rave about how stupid/awful/soul destroying/ridiculous/ostentatious/conformist (delete as appropriate) an iPhone is but the fact of the matter is they are fantastic devices, this is more than evident by their sales figures.
Put it this way - iPhone (9/10) Desire (9.5/10)