I won't answer for bruceo, but it's like the same response from most people looking to make the move from an iPhone but the DROID didn't cut it: UX.
UI/UX, to be specific. People are looking for a satisfying user-experience. The native Android platform is a little rough around the edges and perhaps not as well thought out as the iPhone, which gratifyingly has a simple UI and therefore enjoyable UX. Sense UI is very slick, incorporating a lot of iPhone like gestures, improved connectivity and some small things Android natively did not think of but would rather allow 3rd party developers to cover.
To some, that UX is the end all be all. Some people like to see a huge picture of a contact instead of a line item with thumbnail. Some people like to thumb through their photos instead of clicking on them. Some like to see album covers when then pen through their music. Some like to see their text messages on animated post-it notes. Some like an actual facebook widget that works (native FB widget sucks a duck).
To some, that's necessary because it looks really cool. You can show off to your friends because animation is always cool. bruceo listed some other niggles that he'd rather have that the DROID doesn't include. But that's why you buy an HTC.
At the end of the day, it's UX.